Embryonic DevelopmentVARIATIONS IN EMBRYONIC GERM LAYERS AND BODY CAVITY

Formation of the Digestive Cavity

At the Blastula stage, the embryo forms a fluid filled ball of cells

During Gastrulation, the blastula caves in on one end to form a Gastrula. The cavity formed by gastrulation will form a digestive cavity or digestive tract.

If gastrulation is incomplete, the gastric cavity will have only one opening. This arrangement is characteristic of Cnideria and Platyhelminthes

If gastrulation is complete, the digestive tract will have two openings. This arrangement has 2 variations, protostomes and deuterostomes

Protostomes vs. Deuterostomes

In Protostomes, the blastopore forms the mouth. In Deuterostomes, the blastopore forms the anus and the mouth is the second opening

Embryonic Germ Layers

Organisms that undergo gastrulation fit into 2 categories: diploblasts and triploblasts

Diploblasts only have 2 embryonic germ layers

Gastrulation forms an inner layer of cells (endoderm) and an outer layer (ectoderm)

This diagram shows the endoderm in blue and the ectoderm in black

Embryonic Germ Layers

Triploblasts develop a third germ layer in between the endoderm and the ectoderm

The middle layer is called mesoderm In the diagram, mesoderm is

represented in yellow Variations in the presence, absence

and arrangement of mesodermal tissue is one of the most important distinguishing features of animal phyla

Phylogeny based on Mesodermal Arrangement

Phylum porifera lack a true digestive cavity

Cnideria are diploblastic. They have a digestive cavity but lack mesodermal tissue

The triploblastic phyla vary in regard to the arrangement of mesoderm and body cavity

Development of Mesodermal Tissue

Mesoderm is formed by a migration of endodermal cells into the blastocoel.

Mesoderm forms by one of 2 possible mechanism

The diagram at the right shows mesoderm development in acoelomates and protostomes

The formation of mesoderm initiates near the blastopore. Dividing mesodermal cells migrate into the blastocoel

Acoelomate phyla

Acoelomates do not form a body cavity

Flatworms (Platyhelminthes) are acoelomate

Mesodermal cells fill the entire blastocoel

Internal organs form, but are not separate and distinct

Pseudocoelomate Phyla

Pseudocoelomates have a body cavity, but mesoderm is associated with the outer body wall (ectoderm).

Nematodes and rotifers are pseudocoelomate

All pseudocoelomates have a true digestive tract (2 openings) and are protostomes (blastopore = mouth)

The digestive tract lacks mesodermal tissue, thus is not muscular

Cross Section of a Pseudocoelomate

The diagram shows a cross section through a nematode

Notice the space between organs (body cavity), but note that there is only mesoderm on the outer boundary, associated with the body wall. The Intestine is a simple layer of epithelium (no muscle)

The organs are separate and distinct, but the body cavity is “false” – not fully lined with mesoderm

Coelomate Protostomes

A true body cavity is fully lined with mesodermal tissue.

In coelomate protostomes, the body cavity is “schizocoelous” (note the descriptive root word “schizo”)

The solid mass of mesoderm filling the blastocoel splits due to the programmed cell death (“apoptosis”) of some centrally located cells

The diagram shows the coelom in pink

Coelomate Deuterostomes

All deuterostomes have a true body cavity

In deuterostomes, the blastopore forms the anus (so the “second opening” forms the mouth”)

In deuterostomes, the mesoderm forms from outpouching of the gut

Since it is “caving in” it is “enterocoelous”

Because the coelom is formed from pouches, it is completely surrounded by mesodermal tissue

Schizocoelous vs Enterocoelous