330 T. H. BAST 



is by far the most common method, there is not the least indica- 

 tion of external pressure. The only place where this theory 

 might apply is in the case of protoplasmic fragmentation; that 

 is, protoplasmic masses, with or without small nuclear remains, 

 isolated from the main cell and connected with it by only a nar- 

 row cytoplasmic process (figs. 2 and 8 and fig. 4 previous 

 paper). In such cases the deposition of bone takes place more 

 rapidly on certain surfaces of the cell than on other, thus causing 

 the constriction. 



In reference to the other theories, it should be said that bone 

 cells always multiply by amitosis, and therefore these observa- 

 tions support theory 2. Theory 4 also finds support in these 

 observations in that amitosis occurs in young and rapidly growing 

 bone. The growth is so rapid that the 4-mm. parietal bone of 

 a newborn rat is 9 mm. in diameter at the age of fourteen days. 

 In such rapidly growing tissue it is reasonable that the question 

 of food supply may have considerable influence. Furthermore, 

 in bone the individual cells are connected with the source of 

 food supply by only narrow cytoplasmic processes, which chan- 

 nels may be insufficient to supply the entire cell. While such 

 views seem reasonable, they by no means are final, since any 

 statement regarding function will of necessity be theoretical, 

 and the study of bone cells warrants no positive statement 

 regarding the physiological reason for amitotic cell division. 



In mitosis there is a mechanism, the centrosome, which is 

 apparently directly concerned in cell division. Some would have 

 this mechanism an indispensable factor in all cell division. They 

 regard all other signs of cleavage as incomplete division. Thus, 

 theory 7 admits that the nucleus may divide by amitosis, but 

 that no cytoplasmic division occurs in such cases. The divi- 

 sion of leucocytes however, has been suggested, by adherents, 

 of this view, as an exception to the rule. On the other hand, 

 there are those who hold that complete cell division does occur 

 in amitosis. Jordan, in his paper on ''Amitosis in the epididy- 

 mis of the mouse," has shown that cytoplasmic division follows 

 amitotic division of the nucleus. Those who have described 

 amitosis for sex cells believe in cytoplasmic division after nuclear 

 division. 



