40 



CHORDATE ANATOMY 



correlated with the distribution of yolk in the protoplasm, the larger 

 cells containing the more yolk. The cells of the blastula grade from 



PIGMENT 



NUCLEUS 



Fig. 33. — 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; /, later stage, lateral view. P, polar body. (After 

 Hatschek.) 



minimum size at one pole (animal) of the sphere to maximum size at the 

 pQi_^ opposite pole (vegetal). This 



polarity is established in the egg 

 before cleavage begins. 



In Amphibians. Some amphi- 

 bian eggs (not including the gela- 

 tinous envelope) are about 2 mm. 

 in diameter. Such an egg would 

 possess a volume about eight 

 thousand times that of an egg of 

 Amphioxus. The greater part of 

 the increased bulk is yolk. The 

 egg (Fig. 34) is strongly polarized 

 with reference to the distribution 

 The of the volk in the protoplasm. 

 Development of the Frog's Egg"; The From the animal polc where yolk is 

 Macmiiian Co.) ^^ ^ minimum the quantity in- 



creases toward the opposite vegetal pole where the maximum occurs. 



VEGETAL POLE 



Fig. 34. — Ovarian egg of frog; median 

 section. (Redrawn from Morgan 



