156 BULLETIN OF THE 
and 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 entire absence in the nucleus (Löwit, ’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 Ameba to the very perfect karyokinesis of the 
nearly related Huglypha. 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 seeihs more reasonable to suppose that a process so widely 
extended is inherited, and exists potentially in all cells, rather 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 organic 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*), Löwit 
(91), Verson (’91), Frenzel (’91), and O. 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 Geweben vorkommt, und dass sie zur 
