ANIMAL BIOLOGY. 



[Part I 



whole thing is comparable with what we had in the illustrativi 

 case ; but it is much modified by the presence of yolk-containing 

 cells. The hypoblast, for example, only forms the roof of th< 

 mesenteron, and only gradually grows downwards to line th< 

 sides. 



FIG. 35. FORMATION OF GERMINAL LAYERS. 



i. Section of frog's ovum. ii. Section of upper part of blastodermic 

 vesicle of rabbit, iii. Supervicial view of blastoderm in fowl. iv. Section 

 through, and at right angles to, the primitive groove in rabbit, v. Section 

 through, and at right angles to, the medullary groove. 



Hp. Blastopore. ch. Notochord. ep. Epiblast. Tiy. Hypoblast. In. 

 Inner cells. Ip. Lip of blastopore. m. g. Medullary groove, m. p. Meso- 

 blastic plate, me, Mesoblast. mn. Mesenteron. On. Outer cells, pr. g. 

 Primitive groove, s. c. Segmentation cavity, yk. c. Yolk-containing cells. 



So far we have only got epiblast and hypoblast. But th< 

 invaginated layer which gives rise to hypoblast gives rise als< 

 to mesoblast. For it is not composed of a single layer of cells 

 but of several layers, of which the lowest, that which lines th< 

 roof of the mesenteron, is hypoblast, while the others give ris< 



