248 



Basic Structure of Vertebrates 



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 polarity is established in the egg 

 before cleavage begins. 



In Amphibians. Some amphibian eggs (not including the gelati- 

 nous envelope) are about 2 mm. in diameter. Such an egg would possess 



Fig. 205. Cleavage of egg of Amphioxus. (A) Undi- 

 vided 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. Courtesy, 

 Neal and Band: "Chordate Anatomy," Philadelphia, 

 The Blakiston Company.) 



a volume about 8000 times that of an egg of Amphioxus. The greater 

 part of the increased bulk is yolk. The egg (Fig. 206) is strongly polar- 

 ized with reference to the distribution of the yolk in the protoplasm. 

 From the animal pole, where yolk is at a minimum, the quantity in- 

 creases toward the opposite vegetal pole, where the maximum occurs. 

 Yolk is a nonliving, quite inert substance. The active material in 

 development is protoplasm. The developmental behavior of eggs con- 

 taining much yolk shows quite clearly that the yolk is an impediment 



