1 1 8 CELL-DI VISION 



nuclei was for the purpose of increasing the nuclear surface as an aid 

 to metabolic interchanges between nucleus and cytoplasm. Amitotic 

 division leading to the formation of multinuclear cells is especially com- 

 mon 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 super- 

 ficial layers. Dogiel finds in the lining of the bladder of the mouse 

 that the nuclei of the superficial cells, which secrete the mucus cover- 

 ing the surface, regularly divide amitotically, giving rise to huge mul- 

 tinuclear 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 Infu- 

 soria, which possess two kinds of nuclei in the same cell, a macro- 

 nucleus and a micronucleus. The former is known to be intimately 

 concerned with the processes of metabolism (cf. p. 342). During con- 

 jugation the macronucleus degenerates and disappears and a new one 

 is formed from the micronucleus or one of its descendants. The macro- 

 nucleus is therefore essentially metabolic, the micronucleus genera- 

 tive 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 macronu- 

 cleus than in the micronucleus, and in some cases the former appears to 

 divide directly while the latter always goes through a process of mitosis. 

 These conclusions received a very important support in the work of 

 Vom Rath 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-pro- 

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

 (Randzellen, Stiitzzellen). 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 differen- 

 tiated, 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 conclusion 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." * 



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