THE CENTROSOME 



355 



piece (p. 212), which is itself genetically related to the centrosome of 

 the last cell-generation (p. 170). These facts seem explicable only 

 under the assumption that in these cases the centrosome, or a sub- 

 stance which it carries, gives an active stimulus to the cytoplasm 

 which incites the aster-formation about itself, and in the words of 

 Griffin " disengages the forces at work in mitosis " ('96, p. 174). For 

 these reasons I incline to the view that in the artificial aster-formation 

 described by Morgan ^ the centrosomes there observed should not be 

 regarded as the creations of the asters, but rather as local deposits 

 of material which incite the aster-formation around them. That the 

 centrosomes or astral centres are centres of division (whether active 

 or passive) is beautifully shown by Boveri's interesting observations 

 on " partial fertilization " referred to at page 194. 



Again, Boveri has observed that the segmenting ovum of Ascaris 

 sometimes contains a supernumerary centrosome that does not enter 



Fig. 165. — Kggs of Ascaris with supernumerary centrosome. [BOVERI.] 

 A. First cleavage-spindle above, isolated centrosome below. B. Result of the ensuing division. 



into connection with the chromosomes, but lies alone in the cytoplasm 

 (Fig. 165). Such a centrosome forms an independent centre of divi- 

 sion, the cell dividing into three parts, two of which are normal 

 blastomeres, while the third contains only the centrosome and attrac- 

 tion-sphere. The fate of such eggs was not determined, but they 

 form a complete demonstration that it is in this case the centrosome 

 and not the nucleus that determines the centres of division in the 

 cell-body. Scarcely less conclusive is the case of dis ^grmic eggs in 

 sea-urchins. In such eggs both sperm-nuclei conjugate with the egg- 

 nucleus, and both sperm-centrosomes divide (Fig. 166). The 

 cleavage-nucleus, therefore, arises by the union of tJiree nuclei and 

 foiir centrosomes. Such eggs divide at the first cleavage into four 

 equal blastomeres, each of which receives one of the centrosomes. 



1 Cf. p. 307. 



