THE REPRODUCTION OF THE PROTOZOA 105 



contrary, every possible form of nuclear division is found, from the 

 most simple and direct to karyokinesis as perfect as that seen in 

 the Metazoa. The nuclear division-processes of Protozoa are there- 

 fore exceedingly interesting as furnishing object-lessons in the 

 gradual evolution of the mechanism of nuclear division ; but the 

 extreme diversity in these processes makes it very difficult to deal 

 with them in the Protozoa in a general and comprehensive manner 

 in a short space and without excessive detail. Speaking generally, 

 the indirect nuclear division seen in Protozoa differs from that of 

 the higher organisms in a number of points which indicate that it 

 stands on a lower grade of evolution. As regards the achromatinic 

 elements, the nuclear membrane is usually persistent throughout 

 the process of division, a circumstance which enables a sharp dis- 

 tinction to be drawn between the portions of the division-mechanism 

 derived from the nuclear framework and the cytoplasm respectively. 

 In many cases it is then seen that the cytoplasm does not take any 

 share in the process at all, but that the nucleus divides in a per- 

 fectly autonomous manner, spindle and centrioles remaining intra- 

 nuclear throughout the whole process. As regards the chromatin, 

 the chromosomes when formed are often irregular in form, size, 

 and number ; they often appear imperfectly separated from one 

 another ; they are not always arranged in a definite equatorial plate, 

 but may be scattered irregularly along the spindle ; and they do 

 not always split in the exact manner characteristic of the nuclear 

 divisions of the higher organisms, but divide irregularly and often 

 transversely. 



The principal types of nuclear division in Protozoa will now be 

 described with the aid of a few selected examples. We may begin 

 with those in which the division of the nucleus is autonomous, 

 without co-operation of cytoplasmic elements. 



Division has often been asserted to be direct in cases in which 

 subsequent research has revealed a more elaborate type; never- 

 theless, many typical cases of amitosis occur among Protozoa. In 

 some nuclei of the vesicular type, the chromatin appears to be 

 concentrated entirely in the karyosome, which may contain a 

 centriole also, and when the nucleus divides the karyosome becomes 

 dumb-bell-shaped, and is finally constricted into two halves, the 

 entire nucleus following suit ; as an example of this, almost the 

 simplest conceivable type of nuclear division, may be cited the 

 nuclei of the Microsporidia and allied organisms (Fig. 173, p. 416). 



A type similar in the main to that just described, but slightly 

 more advanced in structural complication, is exemplified by the 

 division of the nucleus in the schizogony of Coccidium (Fig. 51, F M ) ; 

 here there is a peripheral zone of chromatin and a more distinct 

 nuclear membrane. After division of the karyosome, the peripheral 



