250 



ZOOLOGY. 



layers). Throughout almost the whole blastoderm the 

 upper and lower layers are quite distinct in character, but 

 at one point in the margin there is a gradual passage 

 between them : this point marks the future posterior end 

 of the embryo (right-hand of fig. 126). To explain its 

 position in relation to the whole egg we may mention that 

 the future dorsal region of the chick is uppermost, and the 

 broad end of the shell to its left side. 



A comparison of this stage with the blastosphere and 

 gastrula stages of the frog shows us resemblances modified 

 by the excess of yolk. The upper layer cells obviously 

 correspond to the primitive epiblast cells of the frog, and 

 may most conveniently be called epiblast. The lower layer 

 obviously answers to primitive hypoblast. Between the two 



Fig. 126. MEDIAN VERTICAL SECTION OP BLASTODERM. 

 (After Marshall.) 



comes the segmentation cavity, while the sub-germinal 

 cavity answers to the mesenteron. The difference between 

 this stage and that of the frog shown in fig. 118, c, D, is that 

 the mesenteron has a non-cellular mass of yolk for its floor 

 instead of a mass of yolk-laden cells. As this yolk is 

 destined to be absorbed we may fairly say that the mesenteron 

 has no floor, though it has a distinct roof (the layer of 

 primitive hypoblast). In fact, we may compare this stage 

 with the gastrula of Amphioxus (fig. 112, B), if we imagine 

 that (by reason of the enormous yolk) the two layers of 

 cells, instead of being hemispherical, are spread out almost 

 flat, forming a small part of the surface of an enormously 

 larger sphere. 



5. Growth of the Blastoderm. The most striking 

 feature during the early stages of incubation is the extension 



