26 CYTOLOGY 



CHAP. 



break down at the end of the prophase. A single metaphase figure is 

 then formed from the combined chromosomes of the various parts. 



We therefore see that the fact that a nucleus is lobed, or divided into 

 separate parts, is no evidence that it is dividing, or has divided, into 

 daughter nuclei, each of which is capable of proceeding to a separate 

 mitosis. On the contrary, it is probably a temporary phenomenon 

 only, the lobes being withdrawn, or the fragments being fused to form a 

 single nucleus again, at mitosis. 



Irregular or fragmented nuclei are known as polymorphic nuclei, 

 and are of fairly frequent occurrence. Besides the examples cited, they 

 are particularly characteristic of certain leucocytes. 



Even when amitosis is followed by cell division, as appears to be 

 sometimes the case, it is very difficult to show that the nuclei can subse- 

 quently divide mitotically. Meves believes that normal mitosis may follow 

 a peculiar type of amitosis, accompanied by cell division, in the sper- 

 matogonia of the salamander. The amitosis in these cases is not effected 

 by a simple nipping off of a lobe of the polymorphic nuclei described 

 above, but by a more compHcated process in which the centrosome plays 

 an important part, and which has not yet been found to be of general 

 occurrence. The identification of nuclei in which true mitosis is taking 

 place as the descendants of nuclei which have divided amitotically is, 

 however, necessarily uncertain. 



The amitotic multiplication of nuclei in the cleaving egg and germ 

 track of tapeworms was first described by Child (1904), but has been 

 contradicted by others {e.g. Richards, 1909 ; Harman, 1913). A con- 

 siderable controversy has grown up over this matter, for a guide to the 

 Hterature of which the reader is referred to Nakahara (1918). 



An experiment by Nathansohn (1900), following one by Pfeffer, has 

 had a good deal of importance attached to it. So-called amitosis was 

 produced in the green alga Spirogyra by placing the Hving filaments in 

 I per cent ether solution. The process of nuclear di\dsion.was observed 

 under the microscope, and takes twenty-five to thirty minutes. The 

 nucleus, which contains (usually) one large nucleolus, becomes opaque, 

 then presently clears again ; two nucleoU are now found to be present. 

 Then the nucleus becomes constricted and divides, apparently by amitosis, 

 one nucleolus going to each daughter nucleus ; cell division follows. 

 Nathansohn kept these cells under observation and found that mitosis 

 could take place in them, and even that conjugation occurred between 

 descendants of nuclei produced in this way. The above processes were 

 also examined in more detail in fixed and stained preparations. 



Experiments by Hacker (1900), confirmed by Schiller (1909), on the 

 effect of ether on mitosis in the cleaving eggs of the crustacean Cyclops 

 have important bearings on the Spirogyra experiment. Living eggs of 



