No. I.] THE ECHINODERM EGG. 32 I 



I. TOXOPNEUSTES VARIEGATUS. 



A. Observations on the Living Egg. 



I . Axial relations. Paths of the Pronuclei. Polarity. 



The large germinal vesicle occupies an eccentric position, 

 and the polar bodies are formed (in the ovary and before fer- 

 tilization) approximately at the nearest point of the egg- 

 periphery. After the extrusion of the second polar body the 

 egg-archoplasm entirely disappears, while the egg nucleus is 

 re-constituted as a well defined spherical vesicle, which remains 

 near the surface in a very markedly eccentric position cor- 

 responding with that of the original germinal vesicle.^ The 

 axis passing through the egg-nucleus, therefore, corresponds 

 with that passing through the germinal vesicle, and the point 

 at which the polar bodies are formed (Fig. i). 



The spermatozoon may enter at any point with reference to 

 this axis. There is no pre-formed "cone of attraction," but 

 as the spermatozoon enters the ^ZZ^ ^ protoplasmic prominence 

 of the vitellus appears at the point of entrance and inside the 

 vitelHne membrane. This structure, which may be called the 

 "entrance-cone" {''cone d' exsudation" of Fol), persists until 

 about the time the pronuclei unite, and thus forms an unmis- 

 ,takable orienting point for the observation of the sperm-track. 

 The vitelline membrane, formed instantly after attachment 

 of the spermatozoon, carries out with it the tail of the sperma- 

 tozoon attached to its outer surface, and only the nucleus and 

 middle-piece enter the egg. About two or three minutes after 

 the first contact an aster appears near the point of entrance, 

 and in some cases the sperm-nucleus may be seen beside it. 

 Sperm-nucleus and sperm-aster, the latter rapidly growing, 

 now move inwards, and in 5 to 8 minutes (temperature 27° C.) 

 unite with the egg-nucleus which advances to meet them, often 

 undergoing considerable changes of form during its progress. 



1 This was determined by keeping several eggs under continuous observation 

 for several hours. In every case observed the egg-nucleus remained m the im- 

 mediate neighborhood of the polar bodies. It is possible that it may ultimately 

 wander from this position to other parts of the egg, but no evidence of such wan- 

 dering was observed. 



