THE RESTING NUCLEUS 5 



cells the resting stage lasts a much longer time than 

 the few hours required to undergo mitosis, but this is 

 not always so. Some nuclei begin to undergo division 

 and then become arrested, remaining in a particular 

 stage of mitosis for the greater part of their life -cycle. 

 Others may be caused to do so by specific chemical 

 agents such as auramine, sodium cacodylate and 

 colchicine. 1^2 xhe phenomenon normally occurs in 

 some Ciliates of the family Opalinidae, where in 

 different species the nuclei become arrested in meta- 

 phase or anaphase and only subsequently resume 

 mitosis after a long static period. ^^^ Moreover, in 

 Vertebrate oocytes with much yolk (Sharks, Amphibia 

 and Birds) the chromosomes may remain in one stage 

 of meiosis for many months. 2^. 1^4 



This naturally leads to a consideration of the ques- 

 tion : what is it which causes a nucleus to leave the 

 resting stage and enter on the complicated system of 

 changes which we call mitosis ? In certain tissues 

 such as cleaving embryos and frequently in lobules of 

 the testis, division takes place synchronously in all 

 the cells — that is to say every nucleus will be in 

 exactly the same stage at any given moment. On 

 the other hand in epithelia such as the skin and the 

 gut-lining isolated cells enter on mitosis quite sporadic- 

 ally and independently of the neighbouring cells. In 

 the first case we appear to have a ' tissue- control ' of 

 mitosis and in the second a ' cellular control '. Prob- 

 ably in synchronously dividing tissues the substance 

 which is inducing mitosis can diffuse freely from cell 

 to cell (perhaps as the result of protoplasmic ' bridges ' 

 between the cells), while in the second case it is 

 unable to do so. 



It must not be assumed that there is a single 

 mitosis-producing agent. Possibly there is, but if so 

 it is as yet undiscovered. But it is certain that a 

 large number of physiological conditions are capable 

 of stimulating cell division and it seems probable 

 that in natural tissues more than one agency may be 



