THE EGG ol 
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. 
