140 I-'ERTILI'/.ATIOX OF THE OVUM 



entrance of the spermatozoon. In the second and more frequent ease 

 (Ascarts, Fig. 65 ; Physa, Fig. 64; Nereis, Fig. 71 ; Cyclops, Fig. 72), 

 the sperm-nucleus penetrates for a certain distance, often to the cen- 

 tre of the egg, and then pauses while the polar bodies are formed. 

 It then conjugates with the reformed egg-nucleus. In this case, 

 the sperm-aster always divides to form an amphiaster before conju- 

 gation of the nuclei, while in the first case the aster may be still 

 undivided at the time of union. This difference is doubtless due 

 merely to a difference in the time elapsing between entrance of the 

 spermatozoon and conjugation of the nuclei, the amphiaster having, 

 in the second case, time to form during extrusion of the polar bodies. 



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

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

 many cases it may be far in advance of the sperm-nucleus. 1 Boveri 

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

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

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

 nuclei^ though the sperm-nucleus afterwards 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 demonstration of the fact that it is the ccntro- 

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

 tilizing element proper (Boveri, '87, 2). We may therefore conclude 

 that the end of fertilization is the union of the germ-nuclei and the 

 equal distribution of their substance, while the active agent in this 

 process is the centrosome. 



The earliest investigators of fertilization, such as Biitschli and Fol, 

 had no knowledge of the centrosome, and hence no clear idea as to 

 the origin of the asters, but Fol stated in 1873 that the asters repre- 

 sented "centres of attraction" lying outside and independent of the 

 nucleus. Oscar Hertwig showed, in 1875, that in the sea-urchin egg 

 the amphiaster arises by the division of a single aster that first 

 appears near the sperm-nucleus and accompanies it in its progress 

 toward the egg-nucleus. A similar observation was soon afterwards 

 made by Fol ('79) in the eggs of Asterias and Sagitta, and in the 

 latter case he determined the fact that the astral rays do not centre 

 in the nucleus, as Hertwig described, but at a point in advance of it, 

 a fact afterwards confirmed by Hertwig himself and by Boveri 

 ('88, i). Hertwig and Fol afterwards found that in cases of poly- 

 spermy, when several spermatozoa enter the egg, each sperm-nucleus 

 is accompanied by an aster, and Hertwig proved that each of these 

 might give rise to an amphiaster (Fig. 75). 



1 Cf. Kostanecki and NYu-r/.ojski, '96. 



