90 NUCLEAR DIVISION IN PROTOZOA 



3. Autogamy.- — The nucleus of a single individual divides into two. 

 Each of these daughter nuclei undergoes reduction divisions, after which 

 they unite. It is extremely doubtful if this process ever occurs. 



4. Pedogamy. — A single individual divides into two. Reduction 

 divisions of the nuclei of these two daughter individuals which are gametes 

 take place, after which the gametes unite and their nuclei fuse. 



5. Parthenogenesis. — Part of the nucleus of a gamete, which normally 

 develops only after union wdth another gamete, is extruded, after which 

 multiplication occurs. There appear to be no convincing records of such 

 a process amongst the Protozoa. 



NUCLEAR DIVISION AMONGST THE PROTOZOA. 



The division of a nucleus which takes place by simple constriction 

 into two parts without formation of chromosomes is known as amitotic 

 division, to distinguish it from mitotic division, in which definite chromo- 

 somes and a spindle, associated with the presence of centrosomes, occur 

 as described above for the nuclear divisions of the cells of higher animals. 

 Between what appears to be true amitosis and mitosis there occur many 

 gradations. In some cases the appearances are in every way comparable 

 with what has been described above as typical mitosis in the cells of 

 higher animals. In other instances the nuclear membrane persists, and 

 the whole process of mitosis occurs within the nuclear membrane. In 

 other cases, again, there appear to be no centrosomes associated with 

 mitosis within the nuclear membrane, though many observers describe an 

 intranuclear structure, called the centriole, which is supposed to function 

 as a centrosome. As regards the nature of this body and its actual 

 existence there is much diiierence of opinion. That the formation of 

 a spindle may occur without definite centrosomes being identifiable has 

 long been recognized in higher plants, so there is no reason to suppose 

 that this may not happen amongst the Protozoa. When mitosis occurs 

 within the nuclear membrane, definite chromosomes may be formed at 

 the equator of the spindle, and these divide into daughter chromosomes 

 in the usual manner. In other cases, though a spindle is formed, the 

 chromatin granules become arranged irregularly upon the spindle fibres 

 without uniting into definite chromosomes. No equatorial plate is formed, 

 and the nucleus merely constricts into two parts. It is possible that in 

 some of these instances of irregularly arranged chromatin granules there 

 are produced a very large number of minute chromosomes which actually 

 divide. In order to distinguish these intermediate types of mitosis from 

 typical mytosis, the term promitosis has been introduced by Nagler (1909). 



A good illustration of complete mitosis is afforded by the nuclear 



