studies on the dynamics of morphogenesis and inheritance in experimental reproduction. viii. dynamic...

STUDIES ON THE DYNAMICS OF MORPHOGENESIS AND INHERITANCE IN EXPERIMENTAL

REPRODUCTION

VIII. DYNAMIC FACTORS IN HEAD-DETERMINATION IN PLANARIA

C. M. C.HILD From the Hull ZoBlogical Laboratory, University of Chicago

TWO FIGURES

In earlier papers (Child '11 b, '11 c, '11 e) attention has been called to the fact that the frequency of head-formation in isolated pieces of Planaria dorotocephala varies with size of piece, region of body and various external conditions which can be controlled experimentally. This difference in capacity, together with the possibility of comparing rates of metabolism in different pieces by the susceptibility method (Child '13 a), affords a means of determining why some pieces produce a head and others do not and since the formation of the head is the first step in the development of the new individual we have an answer to the question why some pieces give rise to new wholes and others do not.

I. THE TIME OF HEAD-DETERMINATION

Since it can be determined experimentally within wide limits in various ways whether a piece of Planaria dorotocephala shall give rise to a head or not (Child 'llb), it is evident that the fate of the piece as regards head formation is not fixedly and finally determined at the time of isolation of the piece from the parent body. The first step in the analysis of the factors concerned in head determination is to find when the determination of the head occurs. This can be readily and very simply done by the following method.

61

62 C. M. CHILD

Short pieces from the middle region of the body, i.e., the posterior region of the first zooid of Planaria dorotocephala, such for example, as ab, bc and cd in figure 1, give rise in 98 to 100 per cent of the cases, under ordinary conditions, to headless forms (Child

Figure 1

'11 b, '11 c), i.e., no outgrowth of new tissue occuis at the anterior and beyond the healing of the wound.

On the other hand, long pieces with anterior ends at the same levels as those of the short pieces (pieces like ae, uf, ce, cf) produce under the same external conditions 98 to 100 per cent of animals with fully developed normal heads.

DYNAMICS OF MORPHOGENESIS 63

It is of course evident that in these cases the length of the piece is in some way a factor in head-formation, but the question how it affects head-formation will be answered later. At present we are concerned merely with the fact that the cells at the levels, a, b, etc., when at the anterior ends of short pieces, do not give rise to heads, but when at the anterior ends of long pieces do give rise to heads. In order to find approximately when head-formation is determined, it is only necessary to prepare a large series of the long pieces, such as af or cf in figure 1, and at different intervals after section to remove short pieces such as ab or cd from the anterior ends of a certain number. We know that such short pieces when completely isolated at once do not develop heads. If heads appear at their anterior ends after they have remained a certain length of time as the anterior regions of long pieces and have then been isolated, it is evident that the formation of heads must have been determined while they were still a part of the long piece. The records of experi- ments will show the character of results obtained:

Series 458. From well fed worms 18 mm. long, about 150 pieces, including the region cf in figure 1, were prepared and from these the following five lots were taken.

Lot I. From the anterior ends of twenty-five of the long pieces, short pieces like cd were removed 15 to 30 minutes after the long pieces were prepared.

Lot 11. From another twenty-five of the long pieces, similar short pieces were cut 3 to 4 hours after the long pieces were prepared.

Lot 111. Twenty-five similar short pieces 7 to 8 hours after first operation.

Lot IV. Twenty-five short pieces 18 hours after first operation. Lot V. Twenty-five short pieces 24 hours after first operation. These five lots of short pieces were allowed to undergo recon-

stitution at a temperature of 20 to 22°C. and when this was com- plete the character of each piece was recorded; the results appear in table 1.

The five types distinguished have been described in earlier papers (Child '11 b, '11 c ) . The normal form possesses a head

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with two eyes and two lateral cephalic lobes, as in figure 1. In the teratophthalmic form the eyes are either partially or wholly united or unequal in size or abnormally placed. The teratomorphic form has a single eye in the median line and the cephalic lobes appear on the front of the head and may be partially or completely united in the median line. In the anopththalmic 'form an anterior outgrowth is present, containing a small abnor- mal ganglionic mass, but eyes do not appear. And finally, the headless form shows no anterior outgrowth except closure of the wound.

The increase in head-frequency in the short pieces is evident from table 1. In Lot I, 80 per cent remain headless and only 8 per cent give rise to heads with eyes of any kind and none are

TABLE 1

normal. But Lot I1 shows that after 3 to 4 hours the anterior ends of the long pieces have been determined as heads to such an extept that their isolation as parts of short pieces prevents head- formation completely in only 28 per cent, while 24 per cent are anophthalmic and the rest form heads with eyes, 4 per cent normal, 32 per cent teratophthalmic and 4 per cent teratomorphic. In Lot 111, where the short pieces were cut off after 7 to 8 hours as parts of the long pieces, only 12 per cent remain headless and 28 per cent are normal and 60 per cent teratophthalmic. In Lot IV, after eighteen hours, 76 per cent are normal and 16 teratophthalmic, and in Lot V, after twenty-four hours, 80 per cent are normal. After 18 hours the head is so fixedly determined in practically all cases that isolation of the head-forming region as part of a short piece cannot prevent head-formation. The approach in

DYNAMICS O F MORPHOGENESIS 65

Controls I

11, 111 IV

the character of the head to the normal, as well as the increase in head-frequency, is also evident. I n Lot I1 only 4 per cent of normal heads appears, but in Lot 111 28 per cent are normal, in Lot IV, 76 per cent and in Lot V, 80 per cent. This series shows that head determinhtion begins almost at once after section and that in most pieces, the head is fixedly determined within seven to eight hours a€ter section (Lot 111, 88 per cent form heads).

The following series gives in general the same results but brings out some points more clearly than the preceding.

TABLE 2

72

2.30-3.00 4 5.30-6.00 4

12 .oo 36

0

FERATOPH- TERATO- THALMIC MOUPHIC

8

ANOPH- THALMIC

12 24

Series 465. From well-fed worms 15 to 16 mm. in length, about 150 long pi'eces like ae (fig. 1) were cut.

Lot I. Twenty-five short pieces (ab, fig. 1) cut immediately after first operation.

Lot 11. Twenty-five short pieces cut Z$ to 3 hours after first operation.

Lot 111. Twenty-five short pieces cut 53 to 6 hours after first operation.

Lot IV. Twenty-five short pieces cut 12 hours after first operation.

All lots kept at 20 to 22°C. The results appear in table 2, in percentages.

The controls, i.e., twenty-five of the long pieces, all form heads, 72 per cent normal and 28 per cent teratophthalmic. On the other hand, Lot I, consisting of twenty-five of the short pieces removed at once from the anterior end of the long piece, develops no heads at all, 96 per cent being headless and 4 per cent dead.

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The contrast in head-frequency between the long and short pieces is strikingly shown in these two lots. It must be remembered that in the two cases the level of the body concerned in head- formation is the same, or as nearly as possible the same, yet in the long pieces 100 per cent of heads are formed and in the short pieces none. It may be added further that in pieces of intermediate length all possible intermediate percentages of head- frequency are found.

Comparison of Lots I to IV of the short pieces shows essentially the same result as table 1 above. In Lot I all remain headless, in Lot 11, 68 per cent, in Lot III,32 per cent and in Lot IV, 4 per cent. The table also shows that not only does the head- frequency increase but that a marked approach to the normal in the character of the head occurs, as the length of time during which the short piece remains as an anterior portion of the long piece increases. In Lot 11, 20 per cent of the pieces form heads with eyes, in Lot 111, 40 per cent, and in Lot IV, 96 per cent.

The only possible conclusion from these two series is that the factors which determine whether an isolated piece shall give rise to a head or not, begin to act almost immediately after the operation; that within 3 hours after section, the determination between ‘head’ and ‘headless’ has already occurred in about 50 per cent of a lot of pieces under the usual conditions and that this determination has occurred in practically 100 per cent within twelve hours after section.

It also appears from the two tables that the determination of the character of the head formed occurs somewhat later than the determination whether a head shall be formed ornot. At 3 hours and even at 6 hours, anophthalmic as well as normal and teratophthalmic forms appear in. considerable percentages (table 2) and even at 12 hours the percentages of normal heads is only half that in the controls. At 18 hours, however, most heads are determined as normal (Lot IV, table 1).

These and other similar series in which worms of approximately the same size and physiological condition were used and in which external conditions are as nearly as possible uniform, all give similar results. Under these conditions, head-formation


is fixedly determined in most pieces within 8 hours after section and in practically all, within 12 hours after section. The determination of the character of the head occurslater. In this way it is possible to discover approximately the time of determination, not only of the head as a whole, but of the cephalic lobes, eyes and preocular region and of the various types of head. For comparable results animals of similar size and condition and similar external conditions are necessary, because the time of head-determination varies both with internal and external factors and in fact can be altered experimentally.

The times obtained in this way, however, represent ihe times when the structures concerned have become so fixedly determined that they cannot be altered even by extreme changes in conditions. There is every reason to believe that in a given piece it is determined whether a head shall form or not some time before that determination becomes so firmly'fixed as to be un- changeable by altered conditions. Tables 1 and 2 show that in a considerable percentage of pieces, head-determination has become fixed within 3 hours after section. In short, there is no question that under the usual conditions head-determination in pieces must occur, or at least begin, almost immediately after section and the conditions existing in the piece during the first two or three hours after section must constitute the most important factors in the process of head-determination.

11. HEAD-FREQUENCY AND DEGREE OF STIMULATION IN PIECES

In the first paper of this series (Child '11 c) it was shown that the frequency of head-formation in pieces under the usual conditions of temperature, etc., decreases with decrease in length and also with increasingly posterior level of the piece within the limits of a single zooid (Child '11 e). In other words, the shorter or the more posterior a piece is, the less likely it is to give rise to a head. In the preceding paper (Child '14 b) it was found that the amount of temporary increase in the rate of metabolism, i.e., of stimulation of pieces resulting from section, increases with decreasing length and increasingly posterior level of the piece within the limits of a single zooid.

68 C. M. CHILD

Bringing these two groups of facts together, we see that frequency of head-formation decreases as stimulation following section increases. This relation seems at first glance somewhat para- doxical, for it means essentially that the lower the rate of metabolism in a piece in general, the more likely it is to give rise to a head, and vice versa, but in this and following papers evidence will be presented to show that this relation holds within certain limits.

It was demonstrated in the preceding section that head- determination occurs within a few hours after section, at the latest, in. other words, during the period of stimulation following section, which lasts for several hours and is followed by a gradual fall in rate of metabolism. We are then forced to the conclusion that a relatively high rate of metabolism in the piece as a whole, acts in some way as a factor inhibiting head-formation. This conclusion appears somewhat revolutionary, for it is generally believed that the development of a new head on a headless piece is a process of replacement of a missing part and that it is determined and controlled by other parts of the piece. But if the facts cited above are correct, the more vigorous the piece and the more capable it is of determining and controlling processes in other parts during the period when head-determination occurs, the less frequently does a head arise from it. This can mean only that the maintenance of the piece and the process of head-formation are in some way opposed or antagonistic to each other, and that the new head is not determined by the piece, but rather in spite of it. This point of view has already been briefly stated, together with some of the evidence on which it is based (Child "13 b), but further consideration is necessary to make clear its important features and its significance for our conception of the individual and its development.

111. THE PROCESS OF HEAD-DETERMINATION

The head arises from cells adjoining the anterior cut surface of the piece, which are so greatly affected by the altered correlative conditions and the presence of the wound that they lose their differentiation more or less completely, begin to divide rapidly and produce the outgrowth of new tissue. From this


mass of new embryonic tissue the new head arises. This head then develops from the earliest stages of morphogenesis, while in other regions of the piece, except at the posterior end, the original structure undergoes more or less alteration but does not completely disappear in a regression to embryonic cells. In figure 2 the region of head-formation is indicated by the shaded region at z, the region of tail-formation by z, and those regions of the piece which retain the chief features of their structure,

We have already seen that the higher the rate of metabolism in g following section, the less likely is a head to arise from 2, and vice versa, and it has been pointed out that this inverse re-

by Y.

Figure 2

lation between head-frequency and rate of metabolism in other regions of the piece, can mean only that the new head develops, not in correlation with and under the control of the piece as a whole, but, so to speak, in spite of it.

In short, the facts already cited, and a large body of evidence still to be presented, point directly to the conclusion that head- formation in a headless piece is not the restitution of a missing part but the first step in the development of a new individual. If this is the case, then head-formation in a piece is essentially the same process as in embryonic development. There the head, or more specifically the cephalic nervous system, is the first region or organ of the individual to become morphologically visible. So far as we can determine, these early stages of head-development are not dependent upon conditions in other regions of the egg or embryo, but the head-region takes the lead in development.

TEE JOURNAL OF EXPERIMENTAL ZOBLOQY. VOL. 17, NO. 1

70 C . M. CHILD

In an earlier paper (Child ’12) the writer has shown that in Planaria reconstitutional development may be inhibited in various degrees by low concentrations of narcotics and that the formation of a head may still occur when all development of other parts is completely inhibited. Moreover, it is a familiar fact that very short pieces of Planaria may give rise to a single head or to biaxial heads without other parts, and the same is true forTubularia and various other forms. These various facts demonstrate that head-formation in Planaria is possible without any correlative influence of other parts, i.e., it is a process of ‘self-diff’er- entiation’ (Child ’13 b, pp. 614-617). On the other hand, there is no evidence favoring the opposite conclusion. Even as regards pieces undergoing reconstitution, there are no facts indi- cating that head-formation is determined by other regions of the piece. We have simply been accustomed to consider that the piece replaces lost parts and so gives rise again to a new whole. But it has often been pointed out that in such forms as Planaria, the piece does not replace exactly the parts lost, bLt a new whole arises by the formation of a head at one end, a posterior end at the other and the reorganization of remaining portions. What actually occurs in these cases is that a new head region begins to develop as the first step in a new individuation and this new head region dominates other parts and determines a reorganization of the old tissues of the piece. Head-formation is not determined by other parts, but it-or more specifically the formation of the cephalic nervous system-represents the fundamental morphogenetic reaction of the specific cellular material.

The existence in Planaria of the inverse relation between head-frequency and rate of metabolism in the piece as a whole, constitutes further important evidence in support of this view and is readily understood from this standpoint. It can mean only that the cells of the region x (fig. 2) give rise to a head except in case the rate of metabolism in the region y is sufficiently high to retard or inhibit this process of “self-differentiation.”

The rate of metabolism in the cells at x, which we may call ‘rate x,’ is probably determined largely by local conditions con- nected with the altered correlative conditions and the presence


of the wound. Under given external conditions, it probably does not differ very greatly in pieces of different lengtll and from different regions. The rate in the region y (rate y), on the other hand, varies, as we have seen, with size and level of body. When rate y is sufficiently high in relation to rate z, the cells at x are prevented from beginning independent development and producing a new head. On the other hand, the lower the rate of y in relation to the rate of 2, the more independent are the cells at z and the more likely to produce a head.

The fact that head-determination in pieces occurs almost immediately after section, shows that the critical period is at the very beginning of the division and growth in the region 2. Evi- dently, the determination whether a head shall arise or not is essentially simply a question whether the region x shall develop with at least a certain degree of independence of the region y, in which case it produces a head, or whether its development shall be inhibited by y.

If this conception of the process of head-determination is correct, then we arrive at a very simple expression for the head- frequency in pieces of different size and from different regions,

rate x viz., head-frequency = - rate y'

We do not know positively whether rate y must actually be higher than rate J: in order to inhibit head-formation, but there is no reason to believe that such a difference isnecessary. It has been pointed out in earlier papers (Child '11 d, '12, '13 c ) that in the intact animal the rate of metabolism decreases from the head region posteriorly. If this is the case and if the effects of section and the local effect of the wound could be eliminated, rate x in a piece should always be higher than rate y. But the region y is a system of correlated parts with conducting paths and metabolic mechanisms fully developed and capable of a relatively high degree of stimulation. The region z, on the other hand, during the first few hours after section (i.e., the period when its fate is determined) is merely a group of celIs without definite mechanism of correlation corresponding to a head. It is possible that the fully developed region y may overbalance and inhibit

72 C. M. CHILD

the few cells at x, even though rate y per unit of weight or volume is not actually higher than rate x. The development of conducting paths, and in general the differentiation of y, must render it more capable of producing correlative effects upon other parts than are the few cells which are undergoing dewerentiation at x. Probably the rate of metabolism, cell for cell, is higher in x than in y or becomes so very soon after section, for the cells at x are those most affected by section. Susceptibility experi- ments indicate that the region I(: usually possesses the highest rate of any part of the piece. But in order to be able to develop in spite of the region y, rate x must in all probability be considerably higher than rate y, and the differences in susceptibility of the regions x and y indicate that this is actually the case. In fact, before the new tissue has developed far enough to permit the distinction between heads and headless forms, we find in general a greater susceptibility in the region z as compared with y, in those pieces which would later show the higher head-frequency.

But whatever particular relation between rate x and rate y which may prove to be necessary for the inhibition of head- formation or the development of a head, the expression head-

rate x frequency = - still serves to indicate the internal conditions rate y which influence head-formation. And not merely head-frequency in general but the frequency of any of the different types of head, normal, teratophthalmic, teratomorphic, anophthalmic, is determined in the same way. It will be shown that all these different types of head represent simply different degrees of retardation of the process of head-formation, whether by external or internal factors. In .very short pieces, where the whole or nearly the whole piece represents region x and region y is absent or very small, head-frequency should become proportional to rate x, and this is actually the case. Any conditions which decrease the rate sufficiently decrease the frequency of head-formation. This point will be considered more fully at another time. If the region 2 once succeeds in beginning its independent course of development it soon becomes the dominant region of the piece (Child '11 d), develops into a head and determines the establish-


ment of a new axial gradient and so the reorganizatidn of the region y.

As regards the region z of figure 2 from which the posterior end develops, the relations to other parts are not as clear as in the case of 2, for the chief visible differences in tail-formation are merely differences in amount of growth. I n general, however, it is evident that development of the posterior end is directly proportional to rate y. But the stimulation following section has no relation to tail formation, because it is merely temporary and the tail is a subordinate part, depending for its veryexist- ence upon correlations with more anterior regions. It can form at any time, whenever rate y is high enough to determine its development. Tail-formation is retarded or inhibited by all depressing conditions, such as low temperature, low concentrations of narcotics (Child '12), etc., but exactly the same conditions may increase head-frequency in the same pieces. The experimental data upon this point will be presented at another time.

IV. THE FACTORS WHICH DETERMINE LOCALIZATION OF THE NEW HEAD

In all longer pieces of Planaria, the head when it forms is localized at the anterior end of the piece. The basis for this localization is the axial gradient: how the gradient determines the localization we have now to consider. Attention has already been called to the fact that when a planarian (earthworm, etc.) is cut in two, the posterior piece is much more strongly stimulated than the anterior. The same relation is evident in contact stimulation. Slight stimulation of a given region produces much more marked effects posterior than anterior to it. 'In short, the whole mechanism of dynamic correlation in Planaria and similar forms is developed on the basis of the axial gradient. Correla- tion between regions of the body is chiefly in the posterior direction, anterior regions being relatively independent of posterior, and posterior regions relatively dependent upon anterior. That metabolic gradients exist in at least some nerves in the lower animals is known (Child '14 a) and there are reasons for be-

74 C. M. CHILD

lieving that conduction occurs more readily and to greater distances in the downward direction of the metabolic gradient. Conduction against the gradient is possible but requires a much stronger stimulus for a given distance. If the nerve impulse is a wave of chemical reaction, as seems now to be demonstrated (Tashiro '13), it is at least probable that in order to travel for any appreciable distance up the original gradient, the impulse must be strong enough to reverse the gradient temporarily as far as it proceeds, while a much weaker impulse may travel down the gradient for long distances. But whether this interpretation is correct or not, the fact remains that in planarians and many other lower forms, nervous and dynamic correlation (Child '11 a, p. 18) in general, is chiefly from more anterior to more posterior regions.

Admitting this fact, it follows that cells at the anterior end of a piece are in general more independent of other regions of the piece than the cells of any other level. It has been pointed out that the conditions necessary for head-formation are : first, embryonic cells of the species, second, physiological or physical isolation from correlative factors, and third, a sufficiently high rate of metabolism. It is evident that the condition of isolation from correlative factors is more completely fulfilled at the anterior end of the piece than elsewhere. At the posterior ehd, on the other hand, the new cells can never become physiologically isolated unless the axial gradient is eliminated, and in such cases we find that a head may actually arise at the posterior end of a piece. And in cases where the gradient is more or less perma- nently reversed, a tail may arise at the anterior end.

Briefly stated, the localization of new head and tail on a piece are due to the existence of the axial gradient. The head, which develops independently of other parts, develops at the anterior end because this is the only region of the piece where a sufficient degree of physiological isolation can possibly occur, as long as the axial gradient persists. The tail, which arises only as a subordinate part dependent upon and determined by more anterior regions, is localized at the posterior end of the piece because the cells in this region cannot become physiologically


isolated as long as the gradient persists, but develop under the dominance of more anterior parts and therefore produce a tail. By eliminating or reversing the gradient we may alter these localizations.

The headless piece represents a condition intermediate between the development of a head and of a tail at the anterior end. When rate y is sufficiently high in relation to rate x, to decrease the original gradient below a certain minimum in the anterior region of the piece, head-formation is simply inhibited, because neither z nor y dominates completely. But if rate y should become sufficiently high, as compared with rate x, to eliminate the old gradient and establish a new one sufficiently steep in the opposite direction in the anterior region of the piece, the region z will become a tail instead of a head. These points will be further discussed at another time on the basis of further experimental evidence.

V. THE FUNDAMENTAL REACTION SYSTEM OF THE SPECIES

Attention has already been briefly directed in other papers (Child '13 b, '13 c) to certain consequences for the problems of inheritance and development of the conception of the organism developed in these studies. If the head-region is a 'self-differ- entiating' system which arises in development independen fly of other parts and if other parts arise only in correlation with a head region or with a part which has already arisen in this way, we are forced to the conclusion that a single fundamental reaction system is the basis of both development and inheritance. The apical region or head region, or in animals which develop a morphologically differentiated nervous system, the cephalic region of the nervous system which is the dominant part of the head, is a closer approach than any other part of the organism to a morphological expression of this fundamental reaction system. In the lower animals, as well as in the plants, we see as a matter of fact that an isolated cell or group of cells capable of development produces an apical region or head except where it is prevented from doing so by correlation with already existing apical regions or heads.

76 C. M. CHILD

The conclusion that the organism consists fundamentally of a single reaction system which is most closely represented morphologically by the apical region or head, or its dominant part, is forced upon us by the facts. Moreover, it is the only conception which enables us to account satisfactorily for the definite, orderly character of development, its progression in general from anterior to posterior regions, the dominance of the growing tip in plants and of the head in animals, and various other morphological and physiological characteristics of organisms. On the other hand, there are no real facts which indicate that the organism consists fundamentally of a multitude of independent determi- nants, factors or entities of any sort. Even the Mendelian phe- nomena do not demonstrate the existence of independent factors as entities, but merely indicate the existence of different capacities in a system, or of a variety of different systems. The localization of the visible realization of a capacity implies nothing as to the localization of the capacity. Any corpuscular theory of heredity and development demands the assumption of an an- thropomorphic mechanism or ‘vitalistic’ principle of some sort for the management of the corpuscles. The conception of a fundamental reaction system as the basis of inheritance and development avoids all these as well as other difficulties; is developed from experimental data and brings into line a great number of facts which are very generally ignored by current theory.

The fundamental reaction system, dominance of the apical region and the axial gradient are all merely different aspects of the same general idea, viz., that the specific protoplasm of any organism consists fundamentally of a single physico-chemical reaction system which we may, if we desire, conceive as made up of a larger or smaller number of fundamentally similar partial systems. This system is the basis of inheritance and its dynamic capacities, the foundation of hereditary characters. The first step in organization and in embryonic development results from the establishment, in one way or another, of some region or portion of this protoplasmic reaction system as a region of higher


rate of dynamic activity. This region dominates development, becomes the apical or head region and determines the axial gradient or gradients, which constitute the dynamic basis of polarity and of individuation. The organization and development of various parts of the organism retts upon a similar basis of fundamental reaction system and dominance and subordination of parts resulting from differences in rate of reaction. Following papers will be devoted primarily to the development of this general conception on an experimental basis and its appli- cation to particular aspects of the problem of experimental reproduction, and secondarily, to the question of its significance for inheritance and development in general.

VI. SUMMARY

1. Whether or not a head shall arise at the anterior end of an isolated piece of Planaria dorotocephala is determined so fixedly during the first six or eight hours after section that head-formation cannot afterward be prevented by conditions which do prevent it when acting immediately after section. Head-determination undoubtedly begins immediately after section.

2. The period during which head-determination occurs is the period of stimulation following section, and in general the more a piece is stimulated by section the less likely it is to produce a head. Head-formation is a process opposed or antagonistic to the maintenance of the piece.

3. The development of a new head on a headless piece is not the restitution of a missing part but the fist step in the development of a new individual. Whether a head shall develop or not depends primarily on whether the cells which give rise to new tissue at the anterior end of the piece become physiologically isolated to a sufficient degree to develop independently of other parts of the piece, or whether other parts prevent this development. In the former case a head arises, in the latter the piece remains headless.

78 C. M. CHILD

4. If the group of cells which gives rise to the embryonic tissue at the anterior end of the piece is designated z, the similar group at the posterior end, x , and the remainder of the piece, y, we may express head-frequency in pieces in a very simple

form, viz.: head-frequency = - Tail-frequency, on the

other hand, is directly proportional to rate y.

rate x rate y'

5. In pieces of considerable length the new head is localized at the anterior end of the piece because the axial gradient determines that the cells at this end are physiologically isolated to a much higher degree than the cells at the posterior end. A group of cells developing independently at a transverse cut surface gives rise to a head, but when developing in subordination to other parts, gives rise to a posterior end. In short pieces, biaxial heads, biaxial tails or reversal of polarity may occur according to the relations between the rates of the regions z, y and z.

6. The only logical conclusion from the data of experiment and observation is that a single fundamental reaction system is the basis of development and inheritance in each species, race or individual. The apical or head region, or the dominant part of that region, represents the fundamental reaction more nearly than any other part of the organism.


LITERATURE CITED

CHILD, C. M. 1911 a Die physiologische Isolation von Teilen des Organismus. Vortr. und Aufs. uber Entwickelungsmech., H. 11.

1911 b Experimental control of morphogenesis in the regulation of Planaria. Biol. Bull., vol. 20, no. 6.

1911 c Studies on the dynamics of morphogenesis and inheritance in experimental reproduction. I. The axial gradient in Planaria doroto- cephrila as a limiting factor in regulation. Jour. Exp. Zool., vol. 10, no. 3. 1911 b Studies, etc. 11. Physiological dominance of anterior over posterior regions in the regulation of Planaria dorotocephala. Jour. Exp. Zool., vol. 11, no. 3. 1911 e Studies, etc. 111. The formation of new soiiids in Planaria and other forms.

1912 Studies, etc. IV. Certain dynamic factors in the regulatory morphogenesis of Planaria dorotocephala in relation to the axial gradient. Jour. Exp. Zool., vol. 13, no. 1. 1913 a Studies, etc. V. The relation between resistance to depressing agents and rate of metabolism in Planaria dorotocephala and its value as a method of investigation. Jour. Exp. Zool., vol. 14, no. 2.

1913 b Certain dynamic factors in experimental reproduction and their significance for the problems of reproduction and development. Arch. Entwickelungsmech., Bd. 35, H. 4.

1913 c Studies, etc. VI. The nature of the axial gradients in Pla- naria and their relation t o antero-posterior dominance, polarity and symmetry. Arch. f . Entwickelungsmech. Bd. 37, H. 1. 1913 d The asexual cycle of Planaria velata in relation to senescence and rejuvenescence.

1914 a The susceptibility gradient in animals. Science, vol. 39, p. 993. 1914 b Studies, etc. VII. The stimulation of pieces by section in Planaria dorotocephala. Jour. Exp. Zool., vol. 16, no. 3.

1913 Carbon dioxide production from nerve fibers when resting and when stimulated; a contribution to the chemical basis of irrita- bility.

Jour. Exp. Zool., vol. 11, no. 3.

Biol. Bull., vol. 25, no. 3.

TASHIRO, S.

Am. Jour. Physiol., vol. 32, no. 2.

studies on the dynamics of morphogenesis and inheritance in experimental reproduction. viii. dynamic...

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