THE FUNCTION OF THE CENTROSOME. 21 7 



virtue of a favorable distribution of the chromosomes these 

 cytoplasmic rays converge towards one point, a dark body — 

 centrosome — may be seen at the point of convergence. 



These experiments show that a chemical stimulus, applied 

 for a short time to the oocyte in Chaetopterus, initiates a series 

 of mitotic changes which extends over a much longer period. 

 The maturation-divisions, reconstitution of the egg-nucleus, and 

 extrusion of the yolk-lobe occur exactly as though the sperm 

 had entered the egg. It is a natural inference from these phe- 

 nomena that, in normal fertilization, the entering sperm stimu- 

 lates these mitotic activities in a similar manner, i.e., by 

 exerting a chemical influence upon the q^^ and not by furnish- 

 ing the egg with special organs of division. Indeed, the cyto- 

 kinetic changes, including the formation of the centrosome, 

 seem rather to be in response to the activity of the nucleus 

 than vice versa. 



This is the more plausible interpretation of those cases in 

 which the entering sperm initiates mitotic division without the 

 participation of the "sperm-centres." According to Wheeler, 

 no middle-piece or centrosome can be distinguished in the 

 sperm of Myzostoma, though the latter initiates the normal 

 mitosis in the Q.g^ exactly as it does in Chaetopterus. More- 

 over, even in the o^g^ of the sea-urchin, Richard Hertwig has 

 shown that strychnine stimulates the production of asters and 

 even of an amphiaster, although he finds no centrosome. 



In the Qgg of Chaetopterus, which has been stimulated by 

 potassium chloride, two consecutive mitoses are completed in 

 the normal fashion, and the abnormal phenomena commence 

 only after the reconstitution of the egg-nucleus. The abnor- 

 malities may, perhaps, be referred to the lack of sperm-chromo- 

 somes, which are necessary to complete the full number in the 

 cell. That the full number of chromosomes is essential to normal 

 cell activity is attested by its constancy in all tissue-cells and 

 by the universal occurrence of "reduction " before fertilization. 

 The continuance of the cell-divisions in the parthenogenetic 

 eggs of Artemia is also, perhaps, due to the fact that the 

 chromosomes of the second polar globule remain in the ^%%, 

 rather than to the fact that the centrosomes persist. 



