MITOSIS AND AMITOSIS. 405 



division is non-differential so far as the chromosomes are con- 

 cerned. 



Since it is usually impossible to see outlines of individual 

 chromosomal vesicles in the resting nucleus, it is not possible 

 to determine whether the constrictions and tabulations of ami- 

 tosis merely separate whole vesicles from one another. If they 

 do the number of chromosomes arising from each of these vesicles 

 in subsequent mitosis should be the same as in the case of karyo- 

 meres formed by the failure of vesicles to unite in the resting 

 stage. 



It is probable from the work of Boveri (1907) and of Macklin 

 (1916) that when amitotic division of the nucleus is followed by 

 mitosis, each nuclear vesicle gives rise to a fraction only of the 

 normal number of chromosomes and that all the nuclear vesicles 

 in a cell taken together give rise to no more than the normal 

 number. Furthermore, the work of Boveri demonstrates that 

 there is no return to the normal number of chromosomes when 

 once a cell contains an abnormal number. Each nuclear vesicle 

 produced by amitotic division is therefore a karyomere, in every 

 way comparable to those produced by incomplete fusion of 

 chromosomal vesicles; it is a fragment of a nucleus and not an 

 entire nucleus, and this is equally true whether all the karyo- 

 meres lie within a single cell, as is usually the case, or whether 

 in some rare instances they may be distributed to separate cells. 

 In Crepidula it matters not how many karyomeres there are in 

 a cell, if there are two and only two centrosomes all the chromo- 

 somes come together into a single plate and there is a normal 

 division and distribution of each of these chromosomes to the 

 daughter cells. 



Therefore in considering the significance of amitosis.it is of the 

 utmost importance to know whether the constriction of the 

 nucleus is followed by a division of the cell body; if it is not, 

 amitosis is not a permanent nuclear division at all but merely a 

 temporary separation of karyomeres which come together again 

 into a unit structure at the next mitosis. It is a significant fact 

 that in most instances amitotic division of the nucleus is not 

 followed by division of the cell body. 



In this connection it is worth while to compare with the 



