No. 2.] OOKINESIS. 249 



of the tail cease as soon as the head of the spermatozoon penetrates 

 fully into the egg membrane. Its further progress, then, is due not 

 to any automatie movements, but solely to centripetal attraction. 

 As Kupffer and Benecke put it, " Es wird angezogen." 



This attraction is felt even before the undulatory movement 

 ceases, as shown by amceboid changes in the head. " Eine 

 Welle erhebt sich am hintern Ende des Kopfes und lauft an 

 demselben bis an die Spitze hin ab, dcrt sich kuglig zusammen- 

 ballend, weicht dann wieder zurlick, um in erneuetem Anlaufe 

 abermals vorzudringen. Dieses Spiel kann sich minutenlang 

 wiederholen. 1st der Kopf bis in die Nahe der inneren Ober- 

 flache der Eihaut gelangt, so sendet derselbe haufig einen feinen 

 Faden pseudopodienartig vor, der den Rest der Strccke durch- 

 setzt" (pp. 13-14)- 



A third fact of great significance is the elongation of the head 

 of the spermatozoon during its passage through the perivitelline 

 space. The nearer it comes to the vitellus the more elongated 

 it becomes, the increase in length amounting in the end to 

 about one-third. These remarkable changes show the attractive 

 influence of the &gg quite as clearly as Fol's " cone of attraction" 

 demonstrates such action for the spermatozoon. 



Taken all in all, Kupffer and Benecke's paper is by far the 

 most important contribution to the evidences of attraction at a 

 distance that has yet appeared, and whoever doubts such action 

 will do well to read carefully their work. 



The explanation which they offer for the phenomena is not 

 one that I can accept, in so far as it refers the attractive influ- 

 ence exerted upon the spermatozoon, not to the vitelline proto- 

 plasm, but to a centripetally moving egg-nucleus. The idea 

 that this egg-nucleus owes its origin to an impulse given to the 

 vitellus by the spermatozoa of course requires no refutation. 

 The method of accounting for the centripetal movement of the 

 egg-nucleus accords fully with the views presented in the fore- 

 going pages. The reason assigned for the failure of more than 

 one spermatozoon to effect an entrance is entirely untenable, as 

 before pointed out (p. 241). 



The " Leitband des Samens," described by Calberia (17, p. 

 458), in the tgg of Petromyzon, is regarded by that author 

 (p. 485) as the functional equivalent of the "cone of attrac- 

 tion" in Asterias, and by Van Beneden (9, p. 373) it is likened 



