20 



INTRODUCTION PEOTOPL ASM CELL 



supposed to be associated only with highly specialized and pathological 



conditions leading inevitably to death. Cells once having Buffered ami- 



totic division were believed not to be capable of thereafter dividing mitot- 



ically. The work of Child (Biol. Bull., 1007) has shown, however, that 



it is probably of very wide occurrence. Instances have been described in 



most of the animal groups, including the vertebrates. Child has shown 



its occurrence in regions of rapid growth, as in various embryonic tissues, 



e.g., blastoderm of chick (Patterson), and where a secretion is elaborated 



or in places of reserve formation. .These facts may be harmonized with 



its occurrence in starving, degenerating tissues on the basis of a common 



x underlying condition, namely, 



relative scarcity of nutritive 



material. Wieman conceives 



\ -.s&^lfeSfa. ^ am itosis as due to scarcity 



I * * * * 'of oxygen supply. 



-** *'_ Where mitosis and amitosis 



\ . * * * (MRMI J 



' ^ are simultaneously present, it 



* ^^ is more frequently the cells 



with the large nuclei, sur- 

 rounded by a considerable 

 amount of undifferentiated "cy- 

 toplasm, that divide by mitosis. 

 The factors underlying amito- 

 sis most probably exert their 

 final effect indirectly through 

 initial influence upon the cen- 

 trosome. The v best experi- 

 mental evidence in favor of 

 this view is supplied by Xathansolm who grew Spirogyra, normally 

 dividing by mitosis, in a 1 per cent, solution of ether in water, when 

 the cells divided amitotically. On transference to pure water, the cells 

 again divided mitotically. The ether seems to have exerted a 'stupe- 

 fying' effect upon the kinoplasm (centrosome material), compelling 

 division by amitosis. Amitosis is now generally conceded to be of 

 wide occurrence under certain conditions and in certain cells, but it is 

 still quite unanimously disbelieved to occur in germ cells. In the lat- 

 ter it has perhaps not yet been certainly demonstrated to occur in cells 

 actually in the germ cycle. In Mammalia amitosis can be demonstrated 

 in the intermediate layers of stratified squamous (skin), transitional 

 (bladder) and certain ciliated (epididymis, Fig. 25) epithelia; in the 



*'*%* 



FIG. 27. SPERMATOCYTE OF PYERIS CRA- 

 TEGI, A BUTTERFLY, SHOWING A CILIUM 

 ATTACHED TO THE CENTROSOME. 

 (After Meves.) 



