40 PROTOZOOLOGY 



as the controlling center of metabolism, evidences point to the con- 

 clusion that the nucleus contains the genes or hereditary factors 

 which characterize each species of Protozoa from generation to gen- 

 eration, 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 morphological components of the protozoan nucleus 

 are the nuclear membrane, chromatin, plastin and nucleoplasm or 

 nuclear sap. Their interrelationship varies sometimes from one de- 

 velopmental stage to another, 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, c, e) consists of a nuclear mem- 

 brane which is sometimes very delicate but distinct, nucleoplasm, 

 achromatin 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 differ- 

 ent 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 solely upon the 

 difference in affinity to nuclear stains which cannot be standardized 

 and hence do not give uniformly the same result. Following Minchin 

 (1912), the term endosome is advocated here to designate one or 

 more conspicuous bodies other than the chromatin granules, present 

 within the nuclear membrane (Fig, 2, b, d). 



