54 BIOLOGY OF THE PROTOZOA 



Coccidia and Gregarinida). In other cases the endosome retains 

 only a little of the chromatin, the bulk of which is present as a 

 dense network in the zone between endosome and membrane 

 (Endavioeba intestinalis, A. crystalligera, etc.). In still other cases 

 the chromomeres are distributed more or less uniformly throughout 

 the nuclear reticulum (Euglypha alveolata, etc.). 



In vesicular nuclei with endobasal bodies the chromatin may be 

 in the form of more or less regular chromomeres uniformly dis- 

 tributed in the nuclear space (Euglejia type), or more or less com- 

 pactly aggregated about the kinetic element (many species of 

 Endamoeba, various flagellates, Coccidia and Myxosporidia, etc.). 

 Or, finally, the chromatin may be in the form of relatively large 

 granules collected in a zone just within the nuclear membrane 

 (e. g., Pelomyxa), or in fine granular form may make up the chief 

 part of the nuclear membrane (Vahlkampfia Umax, Fig. 26). 



1. Chromatin. — Chromatin has been more a conception than a 

 specific thing, the term being used to designate substances which 

 appear under different forms at different phases of cell life. It 

 appears normally in the form of minute granules or chromomeres 

 (chromidiosomes of Minchin) in the resting nucleus, but during 

 division of the nucleus these granules are massed together usually 

 to form characteristic solid and individualized structures, the 

 chromosomes. On a 'priori grounds chromosomes were early 

 regarded as intimately associated with the phenomena of inheritance 

 (Roux, Weismann, Boveri) and the more recent experimental work 

 in genetics has given substantial evidence of the soundness of this 

 early conclusion. 



Our conception of chromatin is based largely upon investigations 

 upon the nuclear substances of Metazoa and the higher plants. In 

 ordinary descriptions, however, the term is often used in a vague 

 sense to include any substance or body which stains with the so- 

 called nuclear stains, i. e., the basic anilin dyes, while direct chem- 

 ical tests to determine the exact chemical composition of chromatin 

 have been made in very few cases. The best of these show it to 

 be composed mainly of nuclein, one of the most complex of protein 

 substances and rich in phosphorus. 1 



Vague as is the conception of chromatin in Metazoa it is even more 

 so in connection with the Protozoa, where little has been done in 

 a concrete way to throw light on the subject, although much has 

 been written about it. 



Many of the granules found in the cell body of a protozoon as well 

 as those within the nucleus, stain with the usual nuclear dyes and 

 their identification as chromatin is a matter requiring knowledge 

 of their history and fate in the cell. It is only within recent years 



1 For :i critical discussion of chromatin, see Wilson, 1925. 



