70 PROBLEMS OF FERTILIZATION 



after a very early stage. Attention has been focused, 

 largely on theoretical grounds, on two constituents in- 

 timately associated with the middle piece of the sper- 

 matozoon, viz., the centrosome and the mitochondria 

 derived from the spermatid. 



a) The sperm centrosome: Shortly after penetra- 

 tion of the spermatozoon in very many animals an 

 aster arises in association with the sperm nucleus 

 (Figs, ^d, 7, 8); it is centered at the base of the sperm 

 head in all cases in which its actual beginning has been 

 traced, and it has therefore been supposed to be caused 

 by the middle piece of the spermatozoon; a central 

 body soon appears in the aster; this is the sperm centro- 

 some, which has been regarded, therefore, as derived 

 from the middle piece; these observations have been 

 correlated with the histogenesis of the spermatozoon, in 

 which it has been shown in many cases that the centro- 

 some (or part of the centrosome) of the spermatid is 

 located in the middle piece. It was therefore concluded 

 that the centrosome of the sperm aster within the egg 

 is derived from the centrosome of the spermatid. It 

 was further determined in a considerable number of 

 cases that the sperm aster by division forms an am- 

 phiaster which produces the first cleavage of the 

 egg (Figs. 7, 8). From reasoning of this kind Boveri 

 deduced his famous theory of fertihzation that the initia- 

 tion of development is due essentially to the introduc- 

 tion of an active division center into an egg devoid of 

 centrosomes, and hence without capacity for division. 

 This conception obviously involves a whole theory of 

 cell division, and reciprocally such a theory should be 

 supported or weakened by the facts of fertilization. 



