50 PROTOZOAN NUCLEUS 



In some Protista, as, for instance, the bacteria and spiroclisetes, it 

 appears that there is no definite structure which can be called a nucleus, 

 though a granular material usually identified with chromatin is presumed 

 to fulfil the functions of the organized nucleus. AlexeiefE (1924a), however, 

 maintains that the granules are not chromatin, but mitochondria. It is 

 an exceedingly difficult matter to give a precise definition of the term 

 nucleus, though every biologist knows that it is a definite circumscribed 

 structure containing chromatin, and that it behaves in a well-recognized 

 manner. The fact that it divides when cell division takes place is one 

 of its most important features, but there are other structures in the 

 cytoplasm which behave in a similar manner. The one feature which is 

 not shared by other bodies in the cell is that during the sexual process 

 the nucleus, or one of its descendants, is able to unite with another 

 nucleus. In other words, a nucleus is potentially capable of karyogamy. 



The majority of Protozoa possess but a single nucleus, except during 

 the process of multiplication, when two or more may be present. Some 

 forms, however, possess two nuclei during- the greater part of their life- 

 history, and are therefore binucleate, while others, again, have many 

 nuclei and are multinucleate. Such binucleate and multinucleate forms 

 may be regarded as individuals in which the nucleus has divided prepara- 

 tory to division of the body, which for some reason or another has been 

 delayed. In the binucleate and multinucleate individuals the nuclei are 

 easily recognized as being of one type. It sometimes happens that when 

 active multiplication by binary fission is taking place, the rate of division 

 of the nucleus exceeds that of division of the cytoplasm, so that temporary 

 multinucleate stages occur. In the case of Trypanoso7na brucei and other 

 pathogenic trypanosomes in laboratory animals, when active multiplication 

 is proceeding, individuals with four or even a larger number of nuclei 

 may be seen (Fig. 160). In such cases, however, the condition is quickly 

 rectified by repeated divisions of the cytoplasm without further division 

 of the nucleus. In most, if not in all, cases there arrives a period in the 

 life-history of multinucleate forms when division of the body takes place 

 and uninucleate individuals are produced. This is well seen in Opalina 

 ranarum of the intestine of the frog, where the organism is multinucleate 

 during the greater part of its life-history, but in the spring uninucleate 

 individuals are produced (Figs. 448 and 449). 



Amongst Euciliata there exists a special type of binuclearity- These 

 Protozoa usually possess two nuclei, which differ from one another not 

 only in size and structure, but also in function. The larger one of the two 

 is known as the macronucleus, and the other as the micronvcleus (Fig. 70). 

 In ordinary division of the organism both nuclei divide, and when the 

 body is split into two parts the two daughter individuals each have two 



