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 

 Grifhn '' 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 oi Ascaris with supernumerary centrosome. [buVERl.] 

 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 dispermic 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 three nuclei and 

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

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



1 Cf. p. 307. 



