254 LILLIE. [Vol. XVII. 



then nowhere, can the theory hold. Moreover, it seems to me 

 that the theory loses all its significance if the view that the 

 centrosome is a unique and persistent organ of the cell should 

 be given up, as now seems necessary, since it has been shown 

 that centrosomes may arise at any place within the cell, by 

 the observations of Mead ('97), Morgan ('96 and '99), Reinke 

 ('94), and Watas6 ('94). 



Fertilization usually accomplishes two purposes : i . It sets 

 in motion the series of developmental phenomena, the entire 

 mechanism for which is in the Qgg, awaiting only the proper 

 occasion or stimulus. This is not effected primarily by the 

 sperm-centrosomes furnishing cleavage centers, as shown above, 

 but probably by some stimulus, chemical or mechanical, ema- 

 nating from the part of the spermatozoon that penetrates.^ 

 2. Fertilization restores the typical number of the chromo- 

 somes. In this we must recognize its chief function and 

 so return to earlier theories of fertilization. The interca- 

 lation of a sexual generation in the series of asexual or par- 

 thenogenetic generations, wherever these may occur alike in 

 plants and animals, demonstrates, as has been so often pointed 

 out, the fundamental significance of amphimixis. We may 

 regard the first function of fertilization, that of setting in 

 motion the series of ontogenetic changes, as an accessory one 

 adapted to secure the fertilization of as many eggs as possible 

 and to prevent recurrent parthenogenesis. This is of course 

 but a modification of Boveri's theory, that the postulated 

 degeneration of the egg-centrosome is an adaptation against 

 parthenogenesis. Perhaps Mead's suggestion, " The inhibition 

 of division would seem to depend upon the metabolic activity 

 peculiar to the cell by virtue of its internal structure," may 

 partly explain the mechanism of this adaptation of the egg- 

 cell (Mead, '98b, p. 218). Or Loeb's view : "The ions and 

 not the nucleus in the spermatozoon are essential to the 

 process of fertilization" ('99, p. 137). 



^ The universal, or almost universal, occurrence of sperm-asters might thus be 

 interpreted simply as the first response of the egg-cytoplasm to this stimulus. 



