46 BIOLOGY OF THE PROTOZOA 



In a moving protozoon substances of different kinds are constantly 

 involved and take part in the vortex of reactions. Many of these 

 become centers of special activity in the single-celled organism the 

 protoplasm of which is specialized or differentiated to this extent. 

 Such centers, usually indicated by structural characteristics, by func- 

 tional activity, or by susceptibility to certain dyes are plastids of 

 the cell. Other substances in protoplasm by virtue of the reactions 

 which they have undergone in the maelstrom of vitality become 

 stabile, and no longer take an active part in the chemical and phy- 

 sical activities going on about them. Having gone beyond the 

 plastic or labile state in metabolism, but carried along in the living 

 protoplasm, they may serve a useful function in protection, support, 

 offense, or defense of the organism. Such substances are called 

 meta plastids-. 



C. Plastids of the Protozoa.— Centers of special activity, or 

 plastids, are numerous and varied in Protozoa. Some, like chroma- 

 tin, are present in all unicellular animals; others, like kinetic ele- 

 ments, are most conspicuous in actively' moving forms. Some types 

 like chromoplastids, pyrenoids and stigmata are associated w^ith 

 chlorophyll and autotrophic nutrition. Others like chromidia, 

 volutin, and chondriosomes have obscure functions in the cell and 

 are not yet fully pro^^ed to belong in the categor\^ of plastids. 



1. Chromatin,— Chromatin is 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 (chromi- 

 diosomes of Minchin) in the resting nucleus, but during division of 

 the nucleus these granules are massed together to form character- 

 istic 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 

 ordinar^• descriptions, howe\'er, 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 of nuclein, one of the most complex of protein sub- 

 stances and rich in phosphorus.* 



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 



* For a critical discussion of chromatin, see Wilson, 1925. 



