156 BULLETIN OF THE 



aud no hypothesis has yet been offered that will explain all the known 

 instances. Some of the hypotheses that have been suggested I have 

 already dwelt upon at length ; others, as scantiness of chromatin, and 

 even its entii'e absence in the nucleus (Lowit, '90), seem to me still more 

 inadequate. 



One fact in favor of the independence of the two types of division is 

 the sudden change from mitosis to amitosis, without any visible interme- 

 diate stages. Phylogenetically, this is seen in the abrupt transition from 

 the amitotic division of AmcebcB to the very perfect karyokinesis of the 

 nearly related Euglypha. Ontogenetically, of course, the exchange is far 

 more abrupt. In the conjugation of Infusoria, all divisions of the micro- 

 nucleus are undoubtedly mitotic, while the first (after conjugation) and 

 all subsequent divisions of the macronucleus, itself formed from modified 

 micronuclei, are by direct division. Again, the amitosis of the blasto- 

 dermic nuclei of Blatta (Wheeler, '89) is an abrupt change from the 

 perfect mitosis of segmentation. Other instances are the sudden change 

 from mitosis to amitosis in the layers of stratified epithelium, and in 

 the generations of spermatic cells. 



Another fact in favor of my view is the almost universal distribution of 

 amitosis, and its occurrence in many kinds of cells with widely different 

 functions. It seems more reasonable to suppose that a process so widely 

 extended is inherited, and exists potentially in all cells, ratlier than to 

 look upon it as independently assumed in a multitude of special cases. 

 The latter supposition is opposed to all we know of the transmission of 

 fundamental characters. 



While it is evident that both mitosis and amitosis appeared at a very 

 early period of oi'ganic life, it is impossible to say which appeared first. 

 But, on a priori grounds, we may conclude that the simpler type pre- 

 ceded the more complex. 



Cambridge, September 28, 1891. 



It was not until this paper had gone to press that I had access 

 to the recent communications on amitosis by Flemming ('91"), Lowit 

 ('91), Verson ('91), Frenzel ('91), and 0. vom Rath ('91). In his review 

 of recent work on cell division, Flemming says (p. 139) : " Es ist also 

 nicht nur als feststehend anzusehen, dass Amitose vorkommt, sondern 

 auch, dass sie in normal lebenden Gewebeu vorkomrat, und dass sie zur 



