ON THE METHOD OF CELL DIVISION IN T/ENIA. 1 



A. RICHARDS. 



Within the last few years the question of the significance of 

 amitosis has pushed its way forward with renewed activity. 

 From the classic views of Ziegler and vom Rath, Flemming and 

 others that amitosis may be expected in unicellular organisms, in 

 degenerating and senescent cells, and in highly specialized and 

 pathological tissues opinion in some quarters has departed widely. 



On the one hand the tendency has been to narrow this view. 

 In many of the Protozoa mitosis has been found quite general, at 

 least in some stage of the life cycle, while in certain Rhizopods, 

 as Arcella and Euglypha, where direct division was formerly 

 thought to be the means of reproduction, it is now known that 

 mitosis is the common method. In the case of highly special- 

 ized cells numerous examples are reported in which careful study 

 has shown mitosis as the chief means of division. An example 

 of this tendency is seen in Strasburger's work on the tapetum 

 cells. He found that while the period of mitotic division was 

 very short it was sufficient to account for all the observations 

 that had hitherto been explained on the basis of amitosis. Again 

 the application of improved cytological methods of fixing and 

 staining have thrown into disrepute, to a large extent, the old 

 view of the occurrence of direct division in pathological tissues ; 

 indeed the phenomenon of reduction has been described in cancer 

 cells. On these lines of research, then, the tendency has been 

 to limit our notions of the role of amitosis in nuclear and cell 

 division. 



On the other hand more recently a new line of reasoning has 

 been developed, perhaps more rapidly than the facts warrant. 

 This new line, of which Child is the chief, although not the first 

 exponent, is to the effect that direct nuclear division occurs in 

 rapidly dividing cells and in cases in which an orthodromic or 

 acyclic process is involved. In this connection the statements of 



1 Laboratory of Zoology, University of Wisconsin, June 4, 1909. 



309 



