THE REPRODUCTION OF THE PROTOZOA 101 



nucleate individuals at the end of a longer or shorter vegetative 

 existence during which it may have multiplied frequently by 

 plasmotomy. 



The process of fission must now be considered in more detail, 

 beginning with 



1. Division of the Nucleus. As in the case of the cell-body as 

 a whole, the division of the nucleus is effected in various ways. 

 Probably the most primitive type is that in which the nucleus 

 becomes resolved into chromidia, from which, again, secondary 

 daughter-nuclei are reconstituted. This type of division may be 

 termed " chromidial fragmentation." It is of comparatively rare 

 occurrence, but examples of it are found among Sarcodina and 

 Sporozoa. In Echinopyxis two daughter-nuclei are formed in this 

 way (Hertwig, 66, p. 8). In other cases numerous daughter-nuclei 

 may arise, as in the formation of the nuclei of the microgametes 

 in Coccidium (Fig. 50), where the parent nucleus gives off into the 

 cytoplasm a fine dust of chromidial particles which travel to the 

 surface of the cell and become concentrated at a number of spots 

 to form the daughter-nuclei. 



True nuclear division, in which the parent and daughter-nuclei 

 retain throughout the process their individuality and distinctness 

 from the cytoplasm, must be distinguished clearly from the above- 

 mentioned process of chromidial fragmentation. In the vast 

 majority of cases the nucleus divides into two halves by simple or 

 binary fission, which, as already stated, may be repeated several 

 times before cell-division takes place ; but in a few cases the nucleus 

 divides simultaneously into a number of portions by multiple 

 fission. 



In the cells of Metazoa true nuclear division alone occurs, and 

 may follow one or the other of two sharply-marked types, termed 

 comprehensively direct and indirect. In direct division the nucleus 

 is constricted simply into two parts, without circumstance or 

 ceremony. In indirect division, on the other hand, the nucleus 

 goes through a complicated series of changes, following each other 

 in a definite order and sequence, the whole process being known as 

 karyokinesis or mitosis. In spite, however, of the intricate nature 

 of karyokinetic division, and the variations in matters of detail 

 that it exhibits in different cases, the whole process is perfectly 

 uniform in its general plan, and admits of being described without 

 difficulty in generalized terms. Such a description is found in 

 every textbook of biology at the present time, and need not be 

 repeated here ; it will be sufficient to analyze briefly the more 

 important events that take place. 



In the process of "karyokinesis, the achromatinic elements of the 

 nucleus furnish the active mechanisms, while the chromatin-sub- 



