FERTILIZATION 277 



and tail being left on the egg surface. The process in Nereis as de- 

 scribed by Lillie (1912, 1919) is as follows. The egg of this worm has a 

 tough vitelline membrane, an alveolar cortical layer, many yolk and oil 

 droplets, and a large central germinal vesicle (nucleus). If many 

 spermatozoa are present in the vicinity a large number attach themselves 

 to the egg, but usually all but one are carried away by an outflow of jelly 

 from the alveolae of the cortical layer. This layer now takes the form of 

 a zone traversed by radial protoplasmic plates representing the walls of 

 the alveolae. A transparent "fertilization cone" extends from the inner 

 part of the egg across this zone and touches the membrane at the point 

 where the spermatozoon is beginning to penetrate. The perferatorium 

 pierces the egg membrane and becomes attached to the transparent cone. 

 The latter is now withdrawn, carrying the head of the spermatozoon 

 into the egg with it. Thus it appears that the initiative for the final act 

 of penetration lies with the egg rather than with the spermatozoon. 

 Since only the head enters the egg in Nereis it seems clear that the only 

 necessary portion of the spermatozoon in the actual union is the nucleus : 

 the middle piece and tail are accessory and function only as locomotor 

 organs. 



The immediate visible effects of the entrance of the sperm are seen 

 chiefly in changes in the appearance of the cortical region of the egg. If 

 a vitelline membrane is present, as in vertebrates, a "peri vitelline 

 space" usually appears between the membrane and the egg; and this 

 space may in some cases (frog) be great enough to permit the rotation 

 of the egg within the membrane. In the sea urchin a fertilization mem- 

 brane is formed as the result of fertilization : it first appears at the point 

 where the spermatozoon is attached and spreads over the egg with great 

 rapidity. It seems probable that a delicate membrane already present is 

 raised and thus made more conspicuous. In Ascaris, which is parasitic 

 in the intestine of the horse, this membrane becomes very thick and later 

 acts as a protection against the digestive juices of the host. These 

 cortical changes do not depend upon the actual entrance of the sperma- 

 tozoon into the egg: in Nereis they occur before the slow penetration 

 can be completed, or even if the spermatozoon is shaken loose shortly 

 after penetration has begun. 



In describing the remarkable transformation undergone by the 

 spermatozoon within the egg the behavior of its different organs will for 

 the sake of clearness be considered separately. 



The Nucleus. Immediately after gaining entrance to the egg (Fig. 

 108) the sperm head begins to enlarge and assumes the usual form and 

 structure of a nucleus. Meanwhile it advances toward the egg nucleus. 

 As Lillie points out, both male and female pronuclei pass toward a posi- 

 tion of equilibrium in a cell preparing to divide and consequently meet: 

 the assumption of an attractive force between them is unnecessary. By 



