34 PROTOZOOLOGY 



phase of the metabolism. Aside from the importance as the control- 

 ling center of metabolism, evidences point to the conclusion that the 

 nucleus contains the genes or hereditary factors which characterize 

 each species of protozoans from generation to generation, as in the 

 cells of multicellular animals and plants. 



The nucleus 



Because of a great variety of the body form and organization, the 

 protozoan nuclei are of various forms, sizes and structures. At one 

 extreme there is a small nucleus and, at the other, a large voluminous 

 one and, between these extremes, is found almost every conceivable 

 variety of form and structure. The majority of Protozoa contain a 

 single nucleus, though many may possess two or more throughout 

 the greater part of their life-cycle. In several species, each individual 

 possesses two similar nuclei, as in Diplomonadina, Protoopalina 

 and Zelleriella. In Euciliata and Suctoria, two dissimilar nuclei, a 

 macronucleus and a micronucleus, are typically present. The macro- 

 nucleus is always larger than the micronucleus, and controls the 

 trophic activities of the organism, while the micronucleus is con- 

 cerned with the reproductive activity. Certain Protozoa possess 

 numerous nuclei of similar structure, as for example, in Pelomyxa, 

 Mycetozoa, Actinosphaerium, Opalina, Cepedea, Myxosporidia, 

 Microsporidia, etc. 



The essential components of the protozoan nucleus are the nuclear 

 membrane, chromatin, plastin and nucleoplasm. Their interrela- 

 tionship varies sometimes from one developmental stage to an- 

 other, and vastly among different species. Structurally, they fall in 

 general into one of the two types: vesicular and compact. 



The vesicular nucleus (Fig. 2, a) consists of a nuclear membrane 

 which is sometimes very delicate but distinct, nucleoplasm and 

 chromatin. Besides there is an intranuclear body which is, as a rule, 

 more or less spherical and which appears to be of different make-ups 

 as judged by its staining reactions among different nuclei. It may be 

 composed of chromatin, of plastin, or of a mixture of both. The first 

 type is sometimes called karyosome and the second, nucleolus or 

 plasmosome. Absolute distinction between these two terms cannot 

 be made as they are based upon the difference in affinity to nuclear 

 stains which cannot be standardized and hence do not give uni- 

 formly the same result. Following Minchm and others, the term 

 endosome is advocated here to designate one or more conspicuous 

 bodies other than the chromatin granules, present within the nuclear 

 membrane. 



