THE DIVIDING CELL 329 



certain threads are attached to these daughter chromosomes 

 and shorten and thicken during their retreat or " discession " : 

 always some of the spindle threads remain unbroken and continuous 

 across the equator. 



The daughter chromosomes of either side, on arriving in 

 the neighbourhood of the centrosomes, aggregate together to 

 constitute the daughter nucleus or new nucleus. Meantime the 

 centriole has divided. 1 Cell division may accompany or follow 

 the later stages of the process (h) ; but may be quite independent 

 thereof. Thus many cases are known in Plants {e.g. the 

 endosperm formation of Vascular Plants), Animals {e.g. the 

 earlier stages of segmentation of the oosperm), and Protists {e.g. 

 schizogony of Sporozoa), where the nucleus divides by mitosis 

 repeatedly, so as to give rise to a multinucleate mass of 

 protoplasm — the " apocyte " ; and, on the other hand, such a 

 multinucleate structure may ultimately be resolved into uni- 

 nucleate cells by simultaneous or progressive segmentation of 

 the cytoplasm around the single nuclei, without any mitotic 

 phenomena whatever. We infer that the mitotic process is 

 independent of cell-division. We term the force manifested in 

 this process " mitokinetic force," or "mitokinetism." 



•The function of mitosis has been clearly recognised by all, 

 and is, as expressed clearly by Wilson, that of effecting the 

 "partitive" division of the chromosomes. It is evident that 

 every unit of the original chromatic rod when split contributes 

 one-half to either daughter nucleus — as if, in halving the contents 

 of one's purse, one halved every individual coin. 



The mechanism involved in mitosis has been variously 

 interpreted : Klein, Boveri, Van Beneden, have referred it to the 

 muscular contractility of the threads, pulling the chromosomes 

 to the nearer nucleus. But in all divisions some of the threads 

 remain till the end extending from pole to pole across the 

 equator, and it would be necessary to explain their function,, 

 and to assign one different from the half-length threads supposed 

 to pull : while in many cases, notably in plants, the threads 

 all extend from pole to pole and the chromosomes merely 

 glide along them, so that there can be no question of muscular 

 contractility. This explanation, therefore, fails. 



1 Of course, for simplicity's sake I deal here only with the ordinary nuclear 

 division : in " meiotic " division the function is other ; but the mitotic mechanism. 

 is the same. 



