84 CELL-DIVISION 



common in gland-cells. Thus, Klein has described such divisions in 

 the mucous skin-glands of Amphibia, and more recently vom Rath 

 has carefully described it in the huge gland-cells (probably salivary) 

 of the isopod Anilocra ('95). Many other cases are known. Dogiel 

 ('90) has observed exceedingly significant facts in this field that place 

 the relations between mitosis and amitosis in a clear light. It is a 

 well-known fact that in stratified epithelium, new cells are continually 

 formed in the deeper layers to replace those cast off from the 

 superficial layers. Dogiel finds in the lining of the bladder of the 

 mouse that the nuclei of the superficial cells, which secrete the mucus 

 covering the surface, regularly divide amitotically, giving rise to huge 

 multinuclear cells, which finally degenerate and are cast off. The 

 new cells that take their place are formed in the deeper layers by 

 mitosis alone. Especially significant, again, is the case of the ciliate 

 Infusoria, which possess two kinds of nuclei in the same cell, a 

 macronucleus and a micronucleus. The former is known to be 

 intimately concerned with the processes of metabolism (cf. p. 165). 

 During conjugation the macronucleus degenerates and disappears 

 and a new one is formed from the micronucleus or one of its 

 descendants. The macronucleus is therefore essentially metabolic, 

 the micronucleus generative in function. In view of this contrast it 

 is a significant fact that while both nuclei divide during the ordinary 

 process of fission the mitotic phenomena are as a rule less clearly 

 marked in the macronucleus than in the micronucleus, and in some 

 cases the former appears to divide directly while the latter always 

 goes through a process of mitosis. In view of all these facts and 

 others of like import Ziegler, like Flemming, concludes that amitosis 

 is of a secondary character, and that when it occurs the series of 

 divisions is approaching an end. 



This conclusion received a very important support in the work of 

 vom Ralh on amitosis in the testis ('93). On the basis of a compara- 

 tive study of amitosis in the testis-cells of vertebrates, mollusks, and 

 arthropods he concludes that amitosis never occurs in the sperm- 

 producing cells (spermatogonia, etc.), but only in the supporting cells 

 (Randzellen, Stutzzellen). The former multiply through mitosis 

 alone. The two kinds of cells have, it is true, a common origin in 

 cells which divide mitotically. When, however, they have once 

 become differentiated, they remain absolutely distinct ; amitosis 

 never takes place in the series which finally results in the formation 

 of spermatozoa, and .the amitotically dividing "supporting-cells" 

 sooner or later perish. Vom Rath thus reached the remarkable con- 

 clusion that " when once a cell has undergone amitotic division it 

 has received its death-warrant ; it may indeed continue for a time to 

 divide by amitosis, but inevitably perishes in the end." ('91, p. 331.) 



