190 



FERTILIZATION OF THE OVUM 



while in CJuctopterus and Pi en's the first polar spindle has advanced 

 into the anaphase.^ 



It is an interesting and signiftcant 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 principal types of fertilization. /. 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 9 . 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 ivitJiout union of tJic gcnn- 

 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 it is the centro- 

 some alone that incites division of the egg, and is tJierefore 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 



^ QC p. 181. * Cf. Kostanecki and Wicrzejski, '96. 



