THE IMORPHOLOGY OF FERTILIZATION 57 



It is obvious that from a functional point of view 

 all the environmental relations of the ovum are involved 

 in the character of the cortex; changes affecting its 

 permeability must concern the rate of cellular respira- 

 tion, access of water and electrolytes to the interior, 

 and discharge of substances from the cell — ^in short, 

 conditions that affect its metabolism and hence the 

 rate of developmental processes. We have seen in the 

 case of Nereis that the cortical changes involve a dis- 

 charge of material in the form of a clear jelly from the 

 cortex of the egg and the consequent appearance of a 

 perivitelline space. In most other eggs, if there is a 

 discharge of material it is of such a nature as not to 

 be morphologically distinguishable. It has been inferred 

 in certain cases from the fact that the diameter of the 

 egg appears to be slightly reduced following insemination 

 (see p. 148); however, such measurements are pretty 

 close to the margin of error. 



In practically all eggs a perivitelline space appears 

 between the vitelline membrane and the surface of the 

 egg as a result of insemination. The appearances vary 

 here according as there is, or is not, a definite vitelline 

 membrane prior to fertilization. When such a mem- 

 brane is present, as in all vertebrates for instance, it 

 merely becomes more conspicuous in consequence of 

 the formation of the clear space. In the frog's egg the 

 formation of the perivitelline space, which develops 

 rapidly after insemination, enables the egg to rotate 

 within the membrane in accordance with the spccilic 

 gravity of its constituent parts. In teleost eggs the 

 appearance of the space is accompanied by a clearing 

 of the cortical layer of protoplasm, which has been 



