128 F. M. BALFOUR. 



at first to exert but little influence on the protoplasm, but after 

 the formation of the second polar cell, the radial strias around it 

 become very marked, and the pronucleus rapidly grows in size. 

 When it finally unites with the female pronucleus it is equal in size 

 to the latter. In the case when the impregnation is deferred for 

 four hours the male pronucleus never becomes so large as the 

 female pronucleus. With reference to the effect of the time at 

 which impregnation takes place, Asterias would seem to 

 serve as a type. Thus in Hirudinea, Mollusca, and Nematodes 

 impregnation normally takes place before the formation of the 

 polar bodies is completed, and the male pronucleus is accordingly 

 as large as the female. In Echinus, on the other hand, where 

 the polar bodies are formed in the ovary, the male pronucleus 

 is always small. 



Selenka, who has investigated the formation of the male 

 pronucleus in Toxopneustes yanV^^^a^f^/^, differs incertain points from 

 Fol. He finds that usually, though not always, a single sperma- 

 tozoon enters the egg, and that though the entrance may be 

 effected at any part of the surface, it generally occurs at the point 

 marked by a small prominence where the polar cell was formed. 

 The spermatozoon first makes its way through the mucous enve- 

 lope of the egg, within which it swims about, and then bores 

 with its head into the polar prominence. The head of the 

 ^7 spermatozoon on the entering the egg becomes enveloped by the 

 superficial protoplasm, and travels inward with its envelope, 

 while the tail remains outside. As Fol has described, a delicate 

 membrane becomes formed shortly after the entrance of the 

 spermatozoon. The hekd continues to make its way by means 

 of rapid oscillations, till it has traversed about one eighth 

 of the diameter of the egg, and then suddenly becomes still. 

 The tail in the meantime vanishes, while the neck swells up 

 and forms the male pronucleus. The junction of the male and 

 female pronucleus is described by Fol and Selenka in nearly the 

 same manner. 



Giard gives an account of impregnation which is not easily 

 brought into harmony with that of the other investigators. His 

 observations were made on Psammechinus i>iiliaris. At one 

 point is situated a polar body and usually at the pole opposite to 

 it a corresponding prominence. The spermatozoa on gaining 

 access to the egg attach themselves to it and give it a rotatory 

 movement, but according to Giard none of them penetrate the 

 vitelline membrane which, though formed at an earlier period, 

 now retires from the surface of rhe egg. 



Giard believes that the prominence opposite the polar cells 

 serves for the entrance of the spermatic material, which probably 

 passes in by a process of diffusion. Thus, though he regards the 



