VII AMITOSIS IN PROTISTA 203 



of the products of division of differentiated chromatin particles. In the 

 Protista, however, we must extend the word to cover all cases of nuclear 

 division accompanied or preceded by rearrangements of the chromatin 

 which can be interpreted as having this function of bringing about a 

 qualitative rather than a purely quantitative division of the chromatin, 

 ev^en though they do not result in the formation of regular chromosomes. 



The term amitosis is confined, as in the case of the Metazoa and 

 Metaphyta, to a purely mass division of the nucleus without any attempt 

 at a qualitative equality among the daughter nuclei. 



Amitosis, in the sense just defined, has been described repeatedly 

 in the Protista, both in the vesicular and granular types of nucleus. 

 Undoubtedly, however, the trend of modern research in Protistology 

 is to discover in more and more supposed cases of true amitosis a pre- 

 liminary internal readjustment of the chromatin, which suggests that it 

 undergoes a qualitative rather than a purely quantitative partition. 

 Very often these readjustments do not go so far as the formation of 

 the regular chromosomes which we find universally among the higher 

 organisms, but they in all probabihty represent a primitive form of 

 mitosis. It would seem as well therefore to suspend judgement for 

 the present as to whether purely quantitative mass division of simple 

 nuclei ever does take place. The term " simple " is a necessary qualifica- 

 tion, for the Protozoan nucleus sometimes has a very different composition 

 from that of the higher organisms, and in these cases the problem of 

 " amitosis " bears a different complexion. 



Pure mass division of the macronucleus of Infusoria and Acinetaria 

 does indeed appear to be demonstrated. Especially in the latter group 

 is it difficult to see how the division can be qualitative during the process 

 of bud formation in such a form as Ephelota. Here a process of the 

 macronucleus grows out into the developing bud and then becomes 

 nipped off to form the macronucleus of the bud. The macronucleus in 

 these groups, however, appears to be composed of trophochromatin only. 

 It is at any rate destined only to last during the asexual portion of the 

 organism's life cycle ; before conjugation the macronucleus breaks up 

 and disappears, only the micronucleus taking part in syngamy. In both 

 groups the latter nucleus, in contrast to the macronucleus, divides by 

 mitosis throughout the whole life cycle, thereby retaining its qualitative 

 composition. 



The process of mitosis in the Protista ranges in complexity from the 

 merest indication of a sorting out of the chromatin elements before 

 division of the nucleus as a whole, to a mitosis as perfectly developed 

 as any found in the Metazoa, with fully formed chromosomes, an equatorial 

 j)late, centrosomes and complete achromatic figures. 



As examples of primitive mitosis we may take the nuclear division at 



