ORIGIN OF THE CENTROSOME. 



285 



The question will be asked that, if the centrosome be a 

 purely cytoplasmic structure, is there any instance in which 

 this mode of origin can be directly observed ? 



It is not difficult to observe, in sections of certain cells, that 

 whenever three or more cytoplasmic fibrils meet at a common 

 point, we find a microsome at the point of their junction. From 

 this miniature aster to the normal aster with a more or less 

 conspicuous centrosome, the transition is a gradual one. I 

 have seen in the egg of Macrobdella a series of thirteen asters 

 ranging from the miniature aster, with the microsome in its 

 center, to the normal aster with a veritable centrosome. 



Reinke's recent observation shows a similar series. He di- 

 vides the aster into three kinds. The aster of the normal caryo- 

 kinetic figure he calls the 

 primary mechanic center 

 of the cell (Fig. 10, i); 

 the next smaller aster he 

 calls the secondary me- 

 chanic center (2) ; while to 

 the smallest radial struc- 

 ture of the cytoplasm, 

 with a small microsome 

 in its center, he gives the 

 name of tertiary mechanic 

 center (3). 



In view of such exam- 

 ples, of which many more 

 might be given, it is diffi- 

 cult to maintain that the 

 centrosome, with its 

 sphere, is the unique or- 

 gan. The difference be- 

 tween the primary 



3 



Fig:. 10. The connective tissue cell of Salamander 

 and larva. (/) the centrosome, or the primary mechanic center ; 

 i (2) the secondary mechanic center; (j), the tertiary 



tertiary asters, as ot the mechanic center . _ (A fter Reinke.) 

 centrosome and the mi- 

 crosome, is simply the difference of magnitude, and therefore a 

 difference in degree of development, and not in the kind of 

 material of which they are composed, 



