50 MITOSIS : THE CONSTANCY OF THE CHROMOSOMES 



mechanism and the deceptive appearance that is found at mitosis 

 in the Protista. They account for the difficulty in interpreting 

 mitosis in these organisms and for its earher description as 

 " amitosis," that is, a type of division in which the chromatids of 

 each chromosome were not separated to opposite daughter-nuclei. 

 Extreme examples of this difficulty are seen in Vahlkampfia (cf. Belar, 

 1928 b) and in Amceha (Belar, 1926 h). 



Improvement in technique in the Protista and elsewhere now 

 makes it possible to show that all nuclear division is normally 

 mitotic and that the apparent contradiction of genetic principles in 

 the occurrence of amitosis need no longer be taken seriously. 

 Amitosis, the division of a nucleus without the separation of 

 chromatids, is found only in short-lived tissues, such as the endosperm 

 of seed-plants and the deciduous embryonic membranes of animals 

 when it is frequently unaccompanied by cell-division. It does not 

 therefore give rise to independent and equivalent daughter-nuclei. 



There is one apparent exception, the division of the macro- 

 nucleus in the Infusoria. Here many forms of division are found 

 which can scarcely be reconciled with the principles of mitosis {e.g., 

 Spirostomum, cf. Belar, 1926 b), and others which appear as very 

 degenerate mitoses (e.g., Spirochona). But the macronucleus of 

 the infusorian is in a sense analogous with the body-cell nucleus in a 

 multicellular organism, and like the body cells of A scans which 

 lose part of their chromosomes, it need not carry the whole of the 

 hereditary material, resulting from accurate mitotic division. It 

 cannot give rise to a micronucleus, which alone is capable of repro- 

 duction by conjugation. It is off the germ-track. Its life is of 

 restricted duration, for at intervals in the absence of sexual repro- 

 duction, when it always degenerates, it disappears and is replaced 

 by a new macronucleus derived directly from its sister micronucleus 

 by division and the growth of one of the products (Woodruff and 

 Erdmann, 1914, on Paramcecmm) . Belar (1926 b) is inclined to 

 believe that reported cases of a micronucleus arising from a macro- 

 nucleus are due to the smaller nucleus being temporarily included 

 in the larger and hidden by it. Races genuinely devoid of a micro- 

 nucleus are apparently incapable of undergoing sexual reproduction 

 (Woodruff and Spencer, 1924). 



