CELL-ORGANS OP SEA-URCHIN EGG 561 



described the presence of chromatin in the cytoplasm as a peri- 

 odic one : exerting a definite function, trie extranuclear chromatin 

 appears only during brief but definite periods in the development 

 of the cell. Finally the extranuclear chromatin has been con- 

 sidered as a permanent element in the cytoplasm with a partic- 

 ular trophic function opposed to that of the real nuclear repro- 

 ductive chromatin. The extranuclear chromatin has been 

 invariably derived from the nucleus. The independence of the 

 nucleus and the cytosome seemed, however, to be established so 

 firmly, that the observations upon the chromidial formations in 

 the cytoplasm were received with great scepticism. 



In recent time there were discovered in. the cytoplasm structures 

 analogous to the chromidia, but, in their origin, quite independent 

 of the nucleus. Meves and Duesberg called them mitochondria 

 or plastosomes. They attributed to these structures an im- 

 portance similar to that of the nucleus, since both, in their opin- 

 ion, were bearers of hereditary qualities. Just as a nucleus has 

 always for its genesis a division of a mother-nucleus, so have the 

 plastosomes for their origin a division of a mother-plastosome. 

 They are in other terms, self-propagating units. The discovery 

 of the plastosomes added a new cell-organ to an extensive series 

 already known. 



The cytoplasm, the nucleus, the somato- and idiochromatin, 

 the mitochondria, and finally the centrosome, all these cell-parts 

 were regarded as independent in their existence and genesis, and 

 capable of self-multiplication. The cell seemed to become a 

 series of coexisting, independent, self-propagating units. The 

 lack of firmly established notions either of the nucleus, its chrom- 

 atin and nucleolus, or of the centrosphere, not to mention the 

 more recently discovered plastosomes and chromidia, is greatly 

 due to isolated studies of artificially separated stages of cell 

 development. 



Often, as for example in the question of the centrosome, opinions 

 of prominent biologists seemed to diverge widely. For Boveri 

 the centrosome was an organ, present in all animal cells, an 

 organ, which was transmitted from one cell generation to another 

 by multiplication. For Hertwig the centrosome was a temporary 



