PROTOPLASM AND THE CELL. 9 



into a discussion of the various forms of physiological activity 

 of the cell, but one of its physiological functions, reproduc- 

 tion, must receive special attention in conuection with the 

 remarkable structural changes which accompany it. Since 

 the disproval of the doctrine of spontaneous generation the 

 epigrammatic statement Omnis cellula e celluld has been the 

 watchword of modern histology and embryology, and to-day 

 it finds its complement in a corresponding epigram, Omnis 

 nucleus e nucleo. Every cell at present in existence may be 

 assumed to have descended from some previously existing 

 cell, and the nucleus it contains to be a portion of the nucleus 

 of the ancestral cell. New cells arise by the division of 

 previously existing cells, and each division of the cj'toplasm 

 is accompanied by a division of the nucleus. Not but that 

 under certain conditions a division of the nucleus may 

 occur without a corresponding division of the cytoplasm, 

 multinucleated cells thus arising, and conversely a division 

 of the cytoplasm may possibly in certain cases be inaugu- 

 rated without entailing a division of the caryoplasm ; but, as 

 might be expected from the relation wljich exists between 

 the nucleus and the cytoplasm, the division of the latter 

 is usually preceded by a division of the caryoplasm. 



This latter process may take place in two ways. It may 

 begin as a simple constriction of the nucleus which, becoming 

 deeper and deeper, finally separates o£f a portion of it, a divis- 

 ion of the cytoplasm in a similar manner then occurring, so 

 that each of the new cells thus formed contains a portion of 

 the original nucleus. This method of nuclear division, which ■ 

 is rather rare, occurs for instance in the embryonic mem- 

 branes of the Scorpion and is termed direct or amitotic divis- 

 ion, to distinguish it from the more usual indirect or mitotic 

 method which is accompanied by a series of complicated 

 phenomena to which the general term karyoJcinesis or mitosis 

 is applied. 



Starting with a typical cell, consisting of the various parts 

 mentioned above, the karyokinetic phenomena may be re- 

 garded as affecting two constituents, i.e. the centrosome and • 

 the nuclear chromatin. The centrosome which lies at one 

 pole of the nucleus first divides, the two resulting portions 



