6o 



COMPARATIVE ANATOMY 



respiration and excretion. This hollow spherical shape is retained as 



NUCLEUS 



Fig. 37. — Cleavage of egg of AMPHIOXUS. A, undivided egg; B, in process of first 

 cleavage; C, four-cell stage, lateral view; D, four-cell stage, polar view; E, eight-cell 

 stage, lateral view; F, sixteen-cell stage, lateral view; G, eighty-eight cells, lateral view; 

 H, same stage as G, median section; I, later stage, lateral view. P, polar body. (After 

 Hatschek.) 



cleavage continues (Fig. 37, G-I) until between two hundred and three 

 hundred cells have been formed. This stage of the embryo is called 



the blastula. The name, blasto- 

 coele, is applied to the cavity. 



The second and third cleavages 

 introduce inequality of size among 

 the resulting cells. This inequality 

 persists as cleavage goes on. It is 

 correlated with the distribution of 

 yolk in the protoplasm, the larger 

 cells containing the more yolk. 

 The cells of the blastula grade from 

 minimum size at one pole (animal) 

 of the sphere to maximum size at 

 the opposite pole (vegetal). This 



Fig. 38. — Ovarian egg of Frog; median , .^ . ,,1.1 1 • ,1 



section. (Redrawn from Morgan, The Polanty IS established m the egg 



Development of the Frog's Egg; The before cleavage begins. 

 Macmillan Co.) t * 1 ••l- o i • 



In Amphibians. Some amphi- 

 bian eggs (not including the gelatinous envelope) are about 2 mm. in 

 diameter. Such an egg would possess a volume about eight thousand 



YOUK 



VEGETAL POLE 



