AMITOSIS IN BONE CELLS 331 



In unpublished observations on red blood cells of Necturus, 

 Dr. W. S. Miller, of the University of Wisconsin, has found 

 that these blood cells divide amitotically. This division concerns 

 not only the nucleus, but the cytoplasm as well, thus giving rise 

 to two distinct cells. Doctor Miller has been kind enough to 

 allow me to study this material and verify his findings. 



In the face of these findings, it is impossible to say that amito- 

 sis is a nuclear matter only. The observations on bone cells do 

 not help us in solving this question. Since bone cells form a 

 syncytium, the cytoplasm does not completely separate in the 

 dividing cell. In reality the cytoplasm is continuous, but the 

 constriction is so nearly complete that the cells are regarded as 

 units. Those who believe that amitosis results in incomplete 

 division will find support in the division of bone cells. It seems, 

 however, that a tissue whose cells form a syncytium is not a 

 fitting tissue on which to determine this phase of the question. 



Leaving out all theoretical possibilities which these observa- 

 tions suggest, we can make the following assertions: 



1. Amitosis occurs normally among bone cells. 



2. Amitosis occurs in rapidly growing bone. 



3. Bone cells in which amitosis occurs are specialized cells. 



4. Bone cells apparently do not divide completely, but form 

 a syncytium. 



TYPES OF AJVIITOSIS AND THE ROLE OF THE CENTROSOMES 



M. Nowikoff says, ''Some authors differentiate two types of 

 amitosis. The first begins with an elongated nucleus, which 

 later takes on a biscuit like form. The bridge between the two 

 enlarged nuclear ends becomes finer and finer and finally tears 

 completely. Amitosis of the second type results from the forma- 

 tion of a notch in the nuclear membrane, which becomes deeper 

 and deeper and in this way pinches the nucleus into two halves." 

 Wasielewski recognizes these two types. 



The description of the first agrees with the dumb-bell type 

 described above. Nowikoff claims that it is the only type found 

 in bone and tendon cells. This does not correspond at all with 

 my observations on bone, which show that by far the majority 



