MUSEUM OF COMPARATIVE ZOOLOGY. 499 



and by way of inference makes its existence due to an attractive influ- 

 ence exercised, at a distance (1), upon the yolk by the spermatozoon. 



He thinks that there is no vitelline membrane formed in the case of 

 Petromyzon at the time of fecundation, such as Fol has described for 

 Echinodermata. 



It remains to add that the phenomena of fecundation transpire more 

 rapidly the longer the egg has been removed from the animal, provided 

 it has not meantime suffered the change described above as occurring at 

 the expiration of about twelve hours. 



Balfour ('78") has given a short synopsis of fecundation as observed 

 by Fol, 0. Hertwig, Selenka, Giard, and Calberla. He suggests that the 

 pathological symptoms shown in the embryos reared by Fol and Hert- 

 wig may be due to an imperfection of the eggs, induced by a delay in 

 impregnation, rather than to the entrance of more than a single sperma- 

 tozoon. 



In his paper on " Befruchtung," etc., Von Jhering (78) treats, for the 

 most part in a purely objective manner, of the recent discoveries in 

 maturation and fecimdation of animal ova. He emphasizes (p. 121) the 

 fact that no morphological difference exists between nucleus and pro- 

 nucleus. The acceptance of the idea of a pronucleus, in view of the 

 parthenogenetic development of the starfish, is only possible with 

 the reservation that the female pronucleus does not necessarily need 

 the accession of a male pronucleus in order to become a segmentation 

 nucleus. 



The final paper by Selenka (78") on Toxopneustes variegatus, be- 

 sides contributing minor additional details concerning fecundation, con- 

 tains essential modifications of the preliminary notice. In the first 

 place, the union of the two pronuclei is not accomplished quite so 

 promptly as at first supposed. The first contact only results in a weld- 

 ing which lasts about fifteen minutes, during which the male pronu- 

 cleus grows until it reaches the size of the female pronucleus. During 

 all this time, the boundary between the two remains distinguishable. 

 It seems to have been previously mistaken by Selenka for the beginning 

 of the spindle differentiation. Finally, however, the limit between the 

 two pronuclei entirely disappears. 



He corroborates Auerbach's observations of a rotation of the joined 

 pronuclei so far as to say that the long axis of the elliptical conjugation- 

 nucleus soon becomes oblique to the radius along which the spermato- 

 zoon penetrated. Whether this takes place only in the case where 

 the course of the spermatozoon is predetermined by the existence of a 



