42 



CHORDATE ANATOMY 



as in Amphioxus, becomes a hollow sphere or blastula (Fig. 35£). Its 



cavity (blastocoele) is excentric, occupying approximately the animal 



hemisphere only. Its wall is more than one cell thick. The great thick- 



^,T-,^,«*w,-., ness of the wall of the vegetal hemi- 





Y' 



sphere and the consequent excen- 

 tricity of the blastocele are obviously 

 due to the yolk. 



In Reptiles and Birds. In eggs 

 whose yolk-mass greatly exceeds that 

 of the amphibian egg all the proto- 

 plasm is segregated into a thin plate, 

 the germ-disc, lying on the surface 

 of the relatively enormous mass of 

 yolk (Fig. 36). In such an egg, 

 obviously, there is no mechanism 

 for dividing the yolk. Cleavage is 



Fig. 36. — Cleavage of the germ-disc of 

 the egg of a turtle (Glyptemys insculpta) ; 

 eight-cell stage. The egg-shell is not 



shown. About twice natural size. A, r 1 ^ ^i ^1 r xi. 



albumen; C, the eight-cell blastoderm; Confined tO the protoplasm Ot the 

 F, yolk. (Redrawn from Louis Agassiz, ggrm-disc which, following fertiliza- 

 " Embryology of the Turtle.") ... , ,.^ .,, 



tion of Its nucleus, sphts up rapidly 

 and soon consists of hundreds of small cells forming what is then called the 

 blastoderm lying as a thin plate of cells on the surface of the yolk (Figs. 

 36 and 37). But there is continuity of blastoderm with yolk only around 

 the periphery of the blastoderm. Elsewhere a thin space, the sub- 

 germinal cavity, intervenes between blastoderm and yolk (Fig. 37). 



lY" lY' 



Fig. 37. — Early blastoderm of chick; plane of section passes through center of 

 egg. B, blastocoele (subgerminal or cleavage cavity); C, cells of blastoderm; V, fluid- 

 filled vesicles; F^ yellow yolk, F^ white yolk. Magnified nearly twenty diameters- 

 (Redrawn from Duval, "Atlas d'Embryologie.") 



Comparing this embryo with the blastula stages of Amphioxus and frog, 

 it seems reasonable to interpret it as a blastula whose blastocoele is the 

 subgerminal cavity, while its blastoderm is the animal region and the 

 yolk-mass is the vegetal region of the embryo. This recognition of a 

 blastula stage, comparable to that of Amphioxus, in the development of a 

 reptile or bird would hardly have been possible but for the intermediate 

 condition exhibited by the amphibian with its moderate yolk-mass and 

 total cleavage. 



