Yolk sac

Diverticulum from the posterior wall of the yolk sac--------connecting stalk

Endoderm lining yolk sac

Mesodermal cells in the wall of the yolk sac --------blood cells and vessels = ANGIOGENESIS.

Undifferentiated mesenchymal cells

Blood islands Primitive capillary

Intra-embryonic mesoderm

Primitive heart tube

Ectoderm+

endoderm

Ectoderm+endoderm

Primitive heartPericardial cavity

Pleuroperitoneal canal

Intraembryonic coelom

Derivatives of endoderm germ layer

1. Rapid longitudinal growth of the central nervous system especially brain vesicles.

2. Formation of rapidly growing somites.

Cephalo-caudal Folding:

1 .With development of brain vesicles it bulge into the amniotic cavity ----------------- embryonic disc folds.

2 .This is more pronounced in cephalic end (head fold) and in caudal

end (tail fold).

Bucco-pharyngeal membrane

Heart

become ventral

Results: 1. Communication with the yolk sac constrict--------narrow long duct (the

vitelline duct).2. Large part of yolk sac cavity becomes incorporated in the body of embryo :In head fold------------foregut (begins with buccopharyngeal membrane).In tail fold---------------hingut (end at cloacal membrane).In between the two-------midgut (connected to vitelline duct to yolk sac).

Head fold

Tail fold

cloaca

End results

• Buccopharyngeal membrane ruptures in 4th week (mouth opening).

• Cloacal membrane ruptures at 7th week (anus).

• amniotic fluid enter the foregut through mouth opening and leave through the anal opening.

1. Allantois is incorporated in the body of embryo------cloaca.2. Connecting stalk contains: yolk sac duct+ allantois + umbilical vessels.

Lateral foldingResults from rapid growth of somites -----------flat disc folds laterally (lateral folds) and embryo has a round appearance

Results:

1.Ventral body wall is established except for the a small area where the vitelline duct and connecting stalk are attached.

2. Vitelline duct becomes obliterated and midgut losses connection with yolk sac.

Intraembryonic coelom------------------pleural,

pericardial, and peritoneal cavities •Parietal (somatic) layer of intraembryonic mesoderm----------- forms the parietal layer of serous membrane which line the pleural, pericardial, and peritoneal cavities.

•Visceral (splanchnic) layer of intraembryonic mesoderm --------form the visceral layer of this serous membrane.

Derivatives of endoderm

• Epithelial lining of primitive gut and cloaca.

• Epithelial lining of respiratory tract.

• Parenchyma of : thyroid, parathyroid, liver and pancreas.

• Epithelial lining of urinary bladder and urethra.

• Epithelial lining of middle ear and auditory tube.