120 



THE PROTOZOA 



in the life of the cell. It is probable, also, that the elaboration of 

 the process of karyokinetic division in Protista was an indispensable 

 antecedent to the evolution of multicellular organisms, since for 

 the formation of a tissue it is necessary that all the cells which 

 build it up should be perfectly similar in their constitution and 

 properties, and this condition could only be brought about, prob- 

 ably, by karyokinetic division of the nuclei in the process of cell- 

 multiplication. 



In the foregoing paragraphs we have dealt only with simple 

 (binary) nuclear division, but, as already stated, in some cases the 

 nucleus divides by multiple fission into a number of daughter- 

 nuclei simultaneously. A simple instance of direct multiple division 

 of a nucleus, in which, apparently, no centrioles are present, has 

 been described by Lebedew (93) in the nuclei of Trachelocerca 

 (Fig. 66 ; see also p. 448). In this form partitions are formed 

 within the nucleus between the grains and masses of chromatin, 

 and finally the nucleus becomes segmented into a mulberry -like mass 



of daughter - nuclei, 



, .'. :; T 



-V:-xs>c' .>' - : .: .- XTT^_ which separate from 



one another. 



In most cases, prob- 

 ably, of multiple 

 fission the nucleus 

 contains a centriole, 



:o. DO. j our stages 01 direct nrainpie nssiori in _, i -i 1pn - 111 H- n - 1 -,] p fj t , s ,i nn 

 the nuclei of Trachelocerca phcenicopterus. After and tne multiple US. 

 Lebedew (93). is brought about in a 



manner analogous to 



the formation, of a plasmodium by multiplication of the nucleus 

 in a cell which remains undivided that is to say, the centriole 

 multiplies by fission a number of times without the nucleus as 

 a whole becoming divided. Thus, in a nucleus of the simple 

 protokaryon type, containing at first a single karyosonie and cen- 

 triole, division of these structures may take place within the mem- 

 brane without the nucleus as a whole dividing, so that the nucleus 

 contains finally two or more karyosomes, each containing a cen- 

 triole. The karyosomes are ultimately set free from the nucleus, 

 either by being budded off singly from it, or by the nucleus as a 

 whole breaking up ; then each karyosome becomes the foundation 

 of a new nucleus. Division of this type, which may be termed a 

 multiple promitosis, has been described by Zuelzer (86) in Wag- 

 nerella. In cases where the division of the nucleus is of the karyo- 

 kinetic type, repeated divisions of the centriole result in the forma- 

 tion of a complicated multipolar mitotic figure, leading to a multiple 

 division of the nucleus, as seen in the divisions of the nuclei in the 

 male sporont of Aggregata (Fig. 67), as described by Moroff (94). 



ABC 



FIG. 66. Four stages of direct multiple fission in 



