FERTILIZATION OF THE OVUM 



while in Chcztopterus and Pieris the first polar spindle has advanced 

 into the anaphase. 1 



It is an interesting and significant fact that the aster or amphiaster 

 always leads the way in the march toward the egg-nucleus ; and in 

 many cases it may be far in advance of the sperm-nucleus. 2 Boveri 

 ('87, i) has observed in sea-urchins that the sperm-nucleus may indeed 

 be left entirely behind, the aster alone conjugating with the egg- 



Fig. 96. Diagrams of two principal types of fer.ilization. /. Polar bodies formed after the 

 entrance of the spermatozoa (annelids, mollusks, flat-worms). //. Polar bodies formed before 

 entrance (echinoderms). 



A. Sperm-nucleus and centrosome at c? ; first polar body forming at ? . B. Polar bodies 

 formed ; approach of the nuclei. C. Union of the nuclei. D. Approach of the nuclei. E. Union 

 of the nuclei. F. Cleavage-nucleus. 



nucleus and causing division of the egg without union of the germ- 

 nuclei, though the sperm-nucleus afterward conjugates with one of 

 the nuclei of the two-cell stage. This process, known as " partial fer- 

 tilization," is. undoubtedly to be regarded as abnormal. It affords, 

 however, a beautiful illustration of the view that it is the centro- 

 some alone that incites division of the egg, and is therefore the fer- 

 tilizing element proper (Boveri, '87, 2). 



The foregoing facts lead us to a consideration of Boveri's theory 

 of fertilization, which has for several years formed a central point of 

 discussion. The ground for this theory had been prepared by Oscar 



1 Cf. p. 181. 2 Cf. Kostanecki and Wierzejski, '96. 



