536 GENERAL CONSIDERATION IN PROTOZOA 



zoites have been formed, while schizont is the cell which gives rise 

 to the merozoites. 



Protozoa not Endowed with Eternal Life. It was formerly believed 

 that the unicellular protozoa could continue dividing indefinitely, 

 that they were endowed with eternal youth and eternal life, and that 

 they did not go through a period of maturity and still less through a 

 period of old age, followed by death. It was first shown by Biitschli, 

 Hertwig and Maupas, and Calkins, by very thorough investigations, 

 that protozoa, even under favorable conditions, after a certain 

 number of generations reach a condition of lowered vitality and 

 depression. In consequence of this they are no longer able to propa- 

 gate and die unless certain changes occur which lead to the formation 

 of a germ plasm which permits a rejuvenation by sexual reproduction. 

 One of the important changes indicating maturity and the necessity 

 for sexual rejuvenation and reproduction is the formation of the 

 chromidia. This term, as explained, designates the appearance of 

 chromatin granules derived and expelled from the nucleus into the 

 cytoplasm. Schaudinn has shown that the nuclei of conjugating 

 gametes are developed exclusively from such extranuclear chromatin. 

 Mesnil, therefore, proposed to call them idiochromidia. The forma- 

 tion of the extranuclear chromidia or idiochromidia in protozoa occurs 

 by nuclear transfusion, by dissolution of nuclear parts, or by nuclear 

 fragmentation. 



The facts as to maturity and senility of protozoa show that these 

 low unicellular animal organisms are no more endowed with un- 

 limited youth and unlimited individual life than the higher multi- 

 cellular animals or metazoa. Both in their body possess only one 

 substance which under the proper conditions of sexual union is 

 endowed with the property of uninterrupted propagation that is, the 

 germ plasm. 



Metabolism of Protozoa. Protozoa can in general only live where 

 there is considerable moisture. They may, however, in the encysted 

 condition and under other circumstances, withstand drying out for 

 a shorter or longer time, and then be like the spores or the vegetative 

 form of bacteria under similar conditions, in a state of suspended 

 animation, from which they may come to life again when the necessary 

 amount of water is supplied. There are a few protozoa, which like 

 plants possess chlorophyl, and can derive their food and build up 

 the constituents of their body from very simple compounds forming 

 carbohydrates and proteids from them. However, as an almost 

 invariable rule, protozoa cannot subsist on such simple compounds, 

 but they, like the zoometazoa, require carbohydrates and proteids in 

 order to supply their demand for growth and multiplication. Most 

 protozoa are supplied with organs of locomotion particularly for 

 the purpose of obtaining food. Such organs of locomotion act by 

 producing in a fluid, currents which bring toward the primitive 

 animal organism other small animal organisms, bacteria, yeasts, and 



