62 



COMPARATIVE ANATOMY 



the inequality in size of cells at animal and vegetal poles is much greater 

 than in Amphioxus, another consequence of the greater yolk mass. 



After the third cleavage a cavity appears in the midst of the group 

 of eight cells. As cleavages proceed this cavity enlarges and the embryo, 

 as in Amphioxus, becomes a hollow sphere or blastula (Fig. ^gE). The 

 ,„,., .,^,„,j^^^ frog blastula differs from that of 



Amphioxus. Its cavity (blastocoele) 

 is excentric, occupying approxi- 

 mately the animal hemisphere only. 

 Its wall is more than one cell thick. 

 The great thickness of the wall of 

 the vegetal hemisphere and the 

 consequent excentricity of the blast- 

 ocoele are obviously due to the 

 yolk. 



In Reptiles and Birds. In eggs, 

 such as those of reptiles and birds, 

 where the yolk mass greatly exceeds 

 that of the amphibian egg, the polari- 

 zation with respect to the distribu- 

 tion of yolk reaches its limit in that all 

 the protoplasm is segregated into a thin plate, the germ disc, lying on the 

 surface of the relatively enormous sphere of yolk (Fig. 40). In such an 

 egg, obviously, there is no mechanism for dividing the yolk. Cleavage is 

 confined to the protoplasm of the germ disc which, following fertiUzation 



4, 



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I 

 I 



L 



I 



Y« 



Fig. 40. — 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, 

 albumen; C, the eight-cell blastoderm; 

 F, yolk. (Redrawn from Louis Agassiz, 

 Embryology of the Turtle.) 



lY' lY^ 



Fig. 41. — 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.) 



of its nucleus, splits 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. 40 and 41). But there is continuity 

 of blastoderm with yolk only around the periphery of the blastoderm. 

 Elsewhere a thin space, the subgerminal cavity, intervenes between 

 blastoderm and yolk (Fig. 41). Comparing this embryo with the blastula 

 stages of Amphioxus and frog, it seems reasonable to interpret it as a 



