30 THE DEVELOPMENT OF THE CHICK 



stages with the protoplasmic reticulum that forms the walls of 

 the yolk- vacuoles. The germinal disc increases in extent and 

 thickness, and the peripheral protoplasm disappears over most of 

 the yolk. An inflow of the peripheral protoplasm into the disc 

 appears very probable by analogy with the bony fishes where 

 this process can be studied with great ease. 



The method of formation of the neck of the latebra and the 

 so-called nucleus of Pander, or peripheral expansion of the neck, 

 follows more or less directly from the preceding account: As the 

 circumference of the ovum enlarges, the germinal disc is carried 

 out and leaves behind it a trail in which yellow yolk is not formed. 

 When the ovum is fully grown, the exact boundaries between the 

 protoplasmic germinal disc and the yolk are not determinable. 

 The disc itself is charged with small yolk-granules which grade 

 off very gradually into the white yolk lying around and beneath 



the disc. 



The mode of nutrition of the ovum and the formation of the 

 vitelhne membrane remain to be considered. The nutrition is 

 conveyed from the highly vascular theca follicuh by way of the 

 follicular cells, or membrana granulosa, to the ovum. The nutri- 

 ment enters by diffusion; at no stage is there any evidence of 

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

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

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

 or primordium of the vitelhne 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 dehcate 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 wih 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 fertihzation. It is not definitely 



