GENERAL EMBRYOLOGY 



1623 



layer, except at the caudal or posterior extremity of the embryo. 

 The inner layer constitutes the amnion, whilst the outer layer, 

 which is part of the general or extra-embryonic somatopleure, 

 forms the chorion. 



As the circular groove encroaches from all sides upon the ventral 

 aspect of the embryonic area, the somatopleure of the inner wall of 

 the groove forms the lateral and ventral body-walls of the embryo, 

 and, at the site of the future umbilicus, this somatopleure becomes 

 continuous with that of the amnion. 



The subjacent splanchnopleure meanwhile encroaches upon the 

 blastodermic cavity, and, as a result of this constriction, from all 

 sides, the archenteron is incompletely divided into two unequal 

 parts. The small dorsal compartment gives rise to the mid-gut 

 (primitive gut), and the large ventral compartment constitutes the 

 yolk-sac or umbilical vesicle. These two compartments communi- 

 cate with each other at the site of the constriction by a narrow 

 canal, called the vitelline duct 



or yolk-stalk. The walls of the Fore-Got Mid-Gut Hind Gm 



mid-gut and yolk-sac consist of 

 splanchnopleure, and, like their 

 cavities, they are continuous 

 with one another. 



The mid-gut, which lies along 

 the ventral aspect of the noto- 

 chord, is originally a straight 

 groove called the intestinal 

 groove, which extends towards 

 the cephalic and caudal ex- 

 tremities of the embryo. It 

 is, however, subsequently con- 

 verted into a tube. At first it 

 has only one opening — namely 

 that of the vitelline duct or 

 yolk-stalk — through which it communicates with the yolk-sac or 

 umbilical vesicle. Its cephalic and caudal extremities are closed. 



The fore-gut is formed as a tubular recess or diverticulum from 

 the anterior part of the blastodermic vesicle, and it lies within the 

 downward bend formed by the head of the embryo. Posteriorly it 

 opens into the mid-gut compartment of the blastodermic vesicle at 

 the intestinal orifice of the vitelline duct, the opening being knowTi 

 as the anterior intestinal portal. Anteriorly it is, at this stage, closed 

 by the bucco-pharyngeal membrane, which consists of two layers — 

 an outer or ectodermic, and an inner or entodermic. 



The hind-gut is formed, similarly to the fore-gut, as a tubular 

 recess or diverticulum from the posterior part of the blastodermic 

 vesicle, and it lies within the tail-fold. Anteriorly it opens into 

 the mid-gut compartment of the blastodermic vesicle at the intes- 

 tinal orifice of the vitelline duct, the opening being kno\vn as the 

 posterior intestinal portal. The cephalic and caudal ends of the 



Fig. 673. — Yolk-Sac, Vitelline Duct, 

 AND Embryo (Schematic). 



