146 SCIENCE PROGRESS. 



male and female pronucleus, but both were completely in- 

 dependent of each other. This proved two things : first, 

 that the male pronucleus was able to undergo division on its 

 own account when within the egg protoplasm, as had already 

 been shown in another way by Boveri ; and, secondly, that 

 the stimulus, — whether arising in the first instance from the 

 male nucleus alone, or as the result of the interaction of it 

 with the surrounding protoplasm, — could be transmitted so 

 as to set the other, the female nucleus, dividing. And there 

 is another fact which seems to indicate that the division 

 is, really, the result of the mjitital intei'actioii between the 

 nucleus and the cytoplasm, namely, that a spermatozoid of 

 Echinus, when introduced either into a ripe egg or into a 

 non-nucleated fragment of such an egg, at once begins to 

 divide, but that if introduced into a similar egg, or part of 

 an egg, which is not ripe, no change whatever results. All 

 these facts, I think, militate against the great importance 

 claimed for the centrosome as an actual organ of division, 

 and go to show that the nucleus and cytoplasm as a ivhole 

 are the principals concerned in the process. 



There is a good deal of uncertainty surrounding the in- 

 dividuality of the centrosome in plants. It corresponds to the 

 centrosome in its original sense as used in animals, but per- 

 haps not to the modern extension of it which has been invented 

 to embrace those highly inconvenient structures described 

 as microcentra by Heidenhein^ and others. Sometimes the 

 centrosome is marked off from the surrounding radiations 

 by a clear space, and in this case the resulting sphere is 

 termed the centrosphere. Now, it is probably a significant 

 fact that all the clearest demonstrations of centrospheres 

 have hitherto been met with in the lower plants. In some 

 Diatoms they are said to be so obvious as scarcely to re- 

 quire staining to render them visible. In Sphacelaria, 

 originally described by Strasburger," they are extremely 



^ M. Heideiihein, " D. Hautdriisen, d. Amphihien," Sitz. d. Wiirz. d., 

 Pliys. Med. Gesells., 1893 '> ^Iso, " Ueb. Bau. u. function d. Rriesenzellen im 

 Knochenmark," IViirzburger Verhand/., 1S94. 



^ Strasburger, ScJnvarmsporoi, Gameten, pji. Sperniat. u. d. Wesen 

 d. Be/rucht, 1892. 



