H4 Darwin, and after Darwin. 



(Fig. 29), we see the complex processes of karyokinesis 

 in the first two stages of egg-cell division. But 

 similar processes continue to repeat themselves in 

 subsequent stages ; and this, there is now good reason 

 to believe, throughout all the stages of cell-division, 

 whereby the original egg-cell eventually constructs an 

 entire organism. In other words, all the cells com- 

 posing all the tissues of a multicellular organism, at 

 all stages of its development, are probably originated 

 by these complex processes, which differ so much 

 from the simple process of direct division in the 

 unicellular organisms 1 . In this important respect, 

 therefore, it does at first sight appear that we have a 

 distinction between the Protozoa and the Metazoa of 

 so pronounced a character, as fairly to raise the 

 question whether cell-division is fundamentally identical 

 in unicellular and in multicellular organisms. 



Lastly, the only other distinction of a physiologically 

 significant kind between a single cell when it occurs 

 as a Protozoon and when it does so as the unfertilized 

 ovum of a Metazoon is, that in the latter case the 

 nucleus discharges from its own substance two minute 

 protoplasmic masses (" polar bodies "), which are then 

 eliminated from the cell altogether. This process, 

 which will be more fully described later on, appears 

 to be of invariable occurrence in the caseof all egg-cells, 



1 I say "probably," because analogy points in this direction. As a 

 matter of fact, in many cases of tissue-formation karyokinesis has not 

 hitherto been detected. But even if in such cases it does not occur 

 i. e. if failure to detect its occurrence be not due merely to still remain- 

 ing imperfections of our histological methods, the large number of 

 cases in which it has been seen to occur in the formation of sundry 

 tissues are of themselves sufficient to indicate some important difference 

 between cells derived from ova (metazoal), and cells which have not 

 been so derived (protozoal). Which is the point now under discussion. 



