THE EGG 31 



immigration of solid food particles, let alone entire cells, into 

 the growing ovum. At an early stage a definite membrane is 

 formed between the ovum and the follicular cells, the zona radiata 

 or primordium of the vitelline membrane (Fig. 7). This is 

 pierced by innumerable extremely minute pores which become 

 narrow canals as the zona radiata increases in thickness. The 

 follicular cells and the peripheral layer of protoplasm of the ovum 

 are connected by extremely delicate strands of protoplasm that 

 pass through the pores (Holl). In some way the nutriment of 

 the ovum is conveyed through these strands. 



The discussion as to whether the zona radiata is a product of 

 the ovum itself or of the follicular cells seems to me to be largely 

 academic and will not be summarized here. There seems to be 

 sufficient evidence of a primary true vitelline membrane secreted 

 by the ovum itself, though this may not represent the entire 

 zona radiata of older ova. 



The third phase of ovogenesis, maturation or formation of 

 the polar globules, is transferred to the next chapter, because it 

 is overlapped by the process of fertilization. It is not definitely 

 known if maturation in birds may be completed without fertiliza- 

 tion, but it seems probable that, as in many other animals, the 

 completion of maturation is dependent on the stimulus of fertiliza- 

 tion. It is, however, essentially a process absolutely distinct 

 from fertilization, and in some animals (e.g., echinids) is com- 

 pleted without fertilization. 



