190 



FERTILIZATION OF THE OVUM 



while in Chcetopterns and Pieris the first polar spindle has advanced 

 into the anaphase.^ 



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.^ 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 piincipal types of fertilization. /. Polar bodies formed after the 

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

 entrance (echinoderms). 



A. Sperm-nucleus and centrosome at S; first polar body forming at 9. B. Polar bodies 

 formed; approach of the nuclei. C. Union of the nuclei. Z;i. Approach of the nuclei. jS. Union 

 of the nuclei. F. Cleavage-nucleus. 



nucleus and causing division of the Q.^g tvitJiout union of tJie germ- 

 nuclci, 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 // is the centro- 

 some alo7ie that incites division of the egg, and is therefore the fer- 

 tilizing clement 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 



Cf. p. i{ 



Cf. Kostanecki and Wicrzejski, '96. 



