i AMITOSIS 25 



many cases where amitosis has been described, no proof even that cell 

 division follows has been given, and in these cases there is no evidence, 

 even if mitosis does subsequently take place in the cell, that it is not 

 preceded or accompanied by refusion of the nuclear fragments. In the 

 spermatogonia of many animals the nuclei become deeply lobed or 

 constricted during a prolonged resting period, and occasionally one or 

 more of these lobes becomes nipped off, producing a cell with two or 

 more nuclei, generally of unequal sizes. There is no evidence, however, 

 that cell division follows the nuclear fragmentation, nor do we find any 

 reason to believe that each of the nuclear fragments may proceed to a 

 separate mitosis within the same cell. On the contrary, there is every 

 reason for the belief that the lobing, or fragmentation, is merely a tempo- 

 rary phenomenon, probably correlated with the necessity for increase 



W& 



-fr\ 



FIG. 12. 



Four stages in " amitotic " division in a tendon of a new-born mouse. (Nowikoff, A.Z., 1910.) 



of surface relative to volume, and that the lobes^are withdrawn or the 

 fragments fuse together again before mitosis. In Lepidosiren the 

 spermatogonial nuclei during periods of nuclear inactivity become very 

 irregular and deeply lobed. Preparations for mitosis are only found, 

 however, in approximately spherical nuclei, implying that the lobes 

 have been withdrawn. Meves (1891, 1895) found that the spermato- 

 gonia of the salamander are lobed in the winter. In the spring the lobes 

 are withdrawn and the now regularly rounded nuclei proceed to mitosis. 

 In the cleaving eggs of Triton (Rubaschkin, 1905) the nuclei are often 

 completely divided into two, three or even more separate nuclei, though 

 it is probable that in this case they have not been produced by the 

 fragmentation of a single mother nucleus, but by the separation of the 

 telophase chromosomes into groups, from each of which a separate little 

 nucleus is formed. In prophase, chromosomes are formed in each one 

 of the separate nuclei, which remain isolated till the nuclear membranes 



