PROTOZOA 



23 



From the fact that, in many cases at least, the trophochromatin is 

 periodically destroyed and replaced, and from further facts which we 

 shall cite in discussing the significance of conjugation, it would appear 

 that trophochromatin, or some part of the protoplasm associated with 

 it, in the course of its regulative activity eventually becomes effete 

 and is replaced from the idiochromatin, which is not liable to that fate. 

 Perhaps the possession by protozoa of the facility for this replace- 

 ment, and the lack of such facility in the body cells of metazoa, is the 

 explanation of the fact that protozoa are not subject to the "natural 

 death" which eventually overtakes the body of a metazoon. 



The loss of trophochromatin during the formation of gametes is 

 not to be confused with the reduction division of maturation . Reduction 

 divisions, however, have been seen in members of all classes of the 

 Protozoa, and it may be suspected that a process of this kind occurs 

 in all species in which there is syngamy. Such divisions are sometimes 



A B 



Fig. 22. Arcella. After Swarczewsky. A, Vegetative individual. B, In- 

 dividual in process of budding, chm. chromidium; op. opening of shell; 

 nu. nucleus; sh. shell. 



{Actinophrys, etc.) strikingly similar to those of the Metazoa, but in 

 other cases {Paramecium, etc.) are peculiar. The reduction division 

 usually closely precedes syngamy, as in the Metazoa, but in the 

 Telosporidia and Volvocina it takes place in the first division of the 

 zygote, so that for the whole of the rest of its life history the organism, 

 is haploid. 



In many protozoa there are present in the cytoplasm, scattered or 

 massed into a group, numerous granules which, like chromatin, take 

 basic stains and are known collectively as the chromidium (Fig. 22 A). 

 They appear to arise from the nucleus, and have been said, but 

 probably incorrectly, in some cases to give rise to nuclei by con- 

 densation. They may appear upon occasion or be present through the 

 greater part of the life cycle. Their function is uncertain and probably 

 not always the same. 



