202 CYTOLOGY 



CHAP. 



of nucleus approaches therefore the commoner type of Metazoan nucleus. 

 The vesicular and granular types of nuclei grade into one another, however. 



Many Protista have two nuclei differing greatly in structure, and 

 presumably in function. Two principal types of this hinuclearity are 

 found. 



(i) In the Infusoria, one of the two nuclei (macronucleus) is much 

 larger than the other (micronucleus). The former divides amitoticaUy 

 during reproduction by fission, while the latter exhibits a form of mitosis. 

 Moreover, the macronucleus disintegrates before conjugation, and takes 

 no part in that process. In the exconjugant it is formed anew from the 

 zygote micronucleus. It is thus clear that the macronucleus is a nucleus 

 physiologically of a lower order than the micronucleus, and it is supposed 

 to be concerned exclusively with the somatic functions of the cell. 



(2) In the second type of binuclearity, found in the Trypanosomes 

 and their allies, the two nuclei are called respectively the trophonucleus 

 and the kinetonucleus. (It must be mentioned, however, that the nuclear 

 nature or origin of the " kinetonucleus " has been questioned in many 

 cases.) The former is the larger, and the one most nearly corresponding 

 to the nucleus of uninucleate Protista. The kinetonucleus is in close 

 anatomical relation to the flagellum, and is therefore supposed to be 

 specially concerned with the movements of the animal. 



A general review of binuclearity in the Protista is given by Dobell 

 (1909). 



Compared with the Metazoa and Metaphyta, the modes of nuclear 

 multiplication in the Protista are of bewildering variety. This refers 

 both to the behaviour of the chromatin and to the development of the 

 achromatic figure. The latter we will not consider specially, except to 

 point out that the centrosomes when present in the Protista are not 

 uncommonly intranuclear, as indeed is the whole achromatic figure (in 

 cases where this is identifiable). In such cases the whole mitosis may 

 take place without rupture of the nuclear membrane. Centrosomes 

 are, however, sometimes absent ; e.g. in certain Amoebae. 



As regards the chromatin, we may classify the modes of nuclear 

 multiplication into three principal categories — amitosis, mitosis, and 

 through the intermediation of chromidia formation. 



Before discussing mitosis and amitosis in the Protista we must make 

 quite clear the precise meaning which we attach to each of these terms. 

 The word mitosis was coined to describe nuclear division in the Metazoa 

 and Metaphyta, and correctly emphasizes the most significant feature 

 of the process — namely, the linear arrangement of the chromatin elements 

 into the threadlike chromosomes. The function of this is universally 

 assumed to be to faciHtate the exact partition among the daughter nuclei 



