32 PROTOZOOLOGY 



olism which takes place in the cytosome in normal animals, 

 while the cytosome is capable of carrying on catabolic phase of 

 the metabolism. Aside from the importance as the controlling 

 center of metabolism, evidences point to the conclusion that the 

 nucleus contains the genes or hereditary factors which character- 

 ize 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 external body forms and of con- 

 sequent body organizations, 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 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 pos- 

 sesses two similar nuclei, as in Pelomyxa hinucleata, Arcella 

 vulgaris, Diplomonadina, Protoopalina and Zelleriella. In Eucil- 

 iata and Suctoria, two dissimilar nuclei, a macronucleus and a 

 micronucleus, are typically present. The macronucleus is always 

 larger than the micronucleus, and controls the trophic activities 

 of the organism, while the micronucleus is concerned with the 

 reproductive acti\dty. Certain Protozoa possess numerous nuclei 

 of similar structure, as for example, in Mycetozoa, Actino- 

 sphaerium, Opalina, Cepedea, Myxosporidia, Microsporidia, etc. 

 Dileptus anser contains many small macronuclei, a condition not 

 observed in other euciliates. 



The essential components of the protozoan nucleus are the 

 nuclear membrane, chromatin, plastin and nucleoplasm. Their 

 interrelationship varies sometimes from one developmental 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) consists of a nuclear mem- 

 brane which is sometimes very delicate, but distinct, nucleo- 

 plasm and chromatin. Besides there is an intranuclear body which 

 is, as a rule, more or less spherical and which appears to be of dif- 

 ferent make-ups, as judged by its staining reactions among dif- 

 ferent nuclei. It may be composed of chromatin, of plastin, or of a 

 mixture of both. The first type is sometimes called karyosome 



