MATURATION AND IMPREGNATION OP THE OVUM. 127 



elusion that in a large number of instances {Lymnaiis, NepheUs, 

 Cucullanus, Sec), the nucleus in question was formed by the 



Fig. 17. — Ovum of Asterias glacialis, after the coalescence of the male and 

 female pronucleus (copied from Fol). 



fusion of two or more nuclei, and Strasburger at first made a 

 similar statement for F/iallusia, though he has since withdrawn 

 it. Though Biitschli's statements depend, as it seems, upon a 

 false interpretation of appearances, he nevertheless arrived at a 

 correct view with reference to what occurs in impregnation. 

 Van Beneden (3) described in the rabbit the formation of the 

 original segmentation nucleus from two nuclei, one peripheral 

 and the other central, and he gave it as his hypothetical view 

 that the peripheral nucleus was derived from the spermatic 

 element. It was reserved for Oscar Hertwig (11) to describe 

 in Echinus lividus the entrance of a spermatozoon into the egg 

 and the formation from it of the male pronucleus. 



Though there is a general agreement between the most recent 

 observers, Hertwig, Fol, Selenka, Strasburger, &c., as to the 

 main facts connected with the entrance of one spermatozoon into 

 the egg, the formation of the male pronucleus, and its fusion 

 with the female pronucleus, there still exist differences of detail 

 in the different descriptions which partly, no doubt, depend upon 

 the difficulties of observation, but partly also upon the observa- 

 tions not having all been made upon the same species. Hertwig 

 does not enter into details with reference to the actual entrance 

 of the spermatozoon into the egg, but in his latest paper points 

 out that considerable differences may be observed in occurrences 

 which succeed impregnation, according to the relative period at 

 which this takes place. When, in Asterias, the impregna- 

 tion is effected about an hour after the egg is laid and previously 

 to the formation of the polar cells, the male pronucleus appears 



