i6i4 



A MANUAL OF ANATOMY 



Blastopore or Primitive Mouth 



Entoderm . 



Ectoderm 



Fig. 668. — The Gastrula of Am- 

 PHioxus (modified after 

 Wiedersheim). 



The unilaminar blastoderm becomes invaginated at one pole of the blastula, 

 and the original cavity, being thereby gradually encroached upon, ultimately 

 disappears. Another cavity, however, is formed within the invaginated 

 portion of the unilaminar blastoderm, and this cavity is the archenteron or 

 primitive intestinal cavity. The opening by which the archenteron com- 

 municates with the exterior, and which 

 is situated at the place where invagina- 

 tion occurs, is called the blastopore. 

 The result is that a double - walled 

 sacciform body is formed, called the 

 gastrula. The inner wall of the gastrula 

 is formed by the invaginated portion of 

 the unilaminar blastoderm, and this 

 constitutes the entoderm. The outer 

 wall of the gastrula consists of ectoderm. 

 At the margins of the blastopore these 

 two germinal layers are continuous 

 with each other. This constitutes the 

 gastrula-stage or gastrulation. 



In such animals the entoderm is 

 entirely the result of a process of in- 

 vagination. Moreover, the mesoderm is 

 subsequently developed at the margins 

 of the blastopore, and gradually insinu- 

 ates itself between the ectoderm and 

 entoderm. 



In Mammalia no such invagination takes place. The entoderm is formed 

 from cells which are entirely within the blastula or blastodermic vesicle — 

 namely, those which constitute the inner cell-mass. 



Embryonic Area. — ^The bilaminar blastodermic vesicle presents 

 at the upper pole a dark region, which is called the embryonic or 

 germinal disc, or embryonic shield. It is due to a thickening of the 

 embryonic ectoderm, and its shape is at first circular, then oval, 

 and subsequently pyriform, the 



narrow end being directed Neural Fold Neural Groove 



caudalwards. Over its caudal 

 or posterior half a longitudinal 

 streak appears, called the primi- 

 tive streak, which extends for- 

 wards in the median line to the 

 centre of the embryonic area. 

 The streak is due to a prolifera- 

 tion of the deep cells of the 

 ectoderm. Along the primitive 

 streak a furrow is formed, 

 known as the primitive groove, 

 and immediately beneath the 

 primitive streak there is ento- 

 derm. 



As stated, the primitive streak extends only to the centre of the 

 embryonic area. At the anterior end of the streak there is a definite 

 dark spot, which is due to an accumulation of cells on the under 

 aspect of the ectoderm, and is known as Hensen's node. In front 

 of this node there is another dark linear marking, mesially placed 



Head Process 



Hensen's Node 

 Neurenteric Canal 



Primitive Streak 



The Embryonic Area. 



