360 



THE NEW INDIVIDUAL 



Part IV 



Nucleus 



Follicle cells of 

 corona radiata 



Cytoplasm 



Vitelline 

 membra 



Zona 

 pellucida 



Pigmented 



cytoplasm 



Nucleus 



Yolk-rich 

 cytoplasm 



Animal pole 



Vegetal pole 

 B 



Albumen 



Disc of 



protoplasm 



Outer shell 

 membrane 



Air 

 chamber 



Inner shell 

 membrane 



White yolk 



Yellow yolk 



Chalaza 



Shell 

 Vitelline membrane 



D 



Fig. 19.1. Eggs whose size depends upon the amount of food (yolk) that they 

 contain. A, human egg (x200) typical of mammals has practically no yolk and 

 is just visible to the naked eye. At its lower right a human sperm is drawn, very 

 highly magnified, even so its difference in size is striking. B, frog (after T. H. Mor- 

 gan); lower right, a frog's egg surrounded by jelly, natural size. C, hen's egg 

 (after Lillie), abundant yolk; shows disk of protoplasm from which the chick de- 

 velops. D, fly; yolk is in the center of the egg and the embryo forms around it. 

 (Courtesy, Arey: Developmental Anatomy, ed. 5. Philadelphia, W. B. Saunders 

 Co., 1946.) 



lion at the future rear end of the body marks the first ingrowth of the digestive 

 tract. As the cells of the embryo multiply, those of succeeding generations be- 

 come different from their predecessors. The tall cells in the neural folds of the 

 future nerve cord are descendants of low, rounded ones. Groups of cells 

 acquire special shapes and abilities; potential muscle cells gradually come 

 to look and act like muscle. As differentiation goes on cells actually move 

 about, changing their positions, and by so doing affect their neighbor cells 

 and arc affected by them. 



Differentiation transforms the potentiality of the fertilized egg into the com- 

 plex realities of the young animal. In 21 days of incubation the latent power 



