190 



FERTILIZATION OF THE OVUM 



while in CJuctoptenis and Piciis the first jmlar spindle has advanced 

 into the anaphase.^ 



It is an interestinc: 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 he tar 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, moUusks, fiat-worms). II. Polar bodies formed before 

 entrance (echinoderms). 



A. Sperm-nucleus and centrosome at cT ; 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 witJiout union of the gcnn- 

 iijiclei, 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 cy.p. 181. 



Cf. Kostanecki and Wicrzejski, '96. 



