24 BIOLOGICAL LECTURES. 



The spermatozoon usually enters the ovum at about the same 

 time that the first polar body is being formed, though in a con- 

 siderable number of eggs it does not enter until after both 

 polar bodies have been extruded. Soon after its entrance, the 

 sperm nucleus begins to move toward the ^^^ nucleus. This 

 motion is quite slow, several hours (four to eight) being neces- 

 sary to bring the two nuclei together ; it i.s, therefore, possible 

 to find almost every stage in this process in eggs which have 

 been fixed and stained. During the whole of this journey 

 toward the egg nucleus, the sperm nucleus continually increases 

 in size ; but before this nucleus shows any appreciable enlarge- 

 ment, and when it is removed from the egg nucleus by almost 

 the whole diameter of the ovum, the sperm aster appears as a 

 granular sphere, considerably larger than the sperm nucleus, 

 and lying immediately in advance of it. Fig. 4. Throughout 

 the approach of the two })ronuclei, the sperm aster precedes 

 the sperm nucleus, and, as I believe, actually leads it to the 

 egg nucleus. 



As already explained, Boveri believes that the centrosome 

 of the new organism is derived exclusively from the sperma- 

 tozoon. In his latest work (already referred to) he says that, 

 in the case of Ascaris, there is neither centrosome nor aster 

 present in the formation of the two polar bodies, although 

 they were present at an earlier stage in the oogenesis ; he 

 therefore concludes that in this case the centrosomes of the 

 segmentation spindle, which forms later, must be derived 

 exclusively from the sperm centro.some. Vejdovsky^ has 

 reached the same conclusion with regard to one of the anne- 

 lids, Rhinchelmis. He says, regarding l^\)rs communication 

 relative to the ovocenter, that no one has hitherto seen such 

 a body in connection with the female pronucleus, and that he 

 conceives it to be the scarcely functional remnant of the 

 Eikcnipcriplasic, or egg aster. " It is therefore," he says, 

 "questionable to assume the presence of an ovocenter in 

 order to make a general law that in fertilization not only the 

 pronuclei, but also the halves of the ovocenter and spermo- 



1 Vejdovsky. l^emerkungen zur Mitteihing II. Fol's "Contribution a I'liistoire 

 de la fecondation." Auat. Anzciger, Bd. 6, No. 13. 



