26 INVERTEBRATE ZOOLOGY 



the rupture of the original cell wall numerous small cells, called 

 spores, are liberated. 



In the indirect nuclear divisions mentioned above, there are 

 frequently one or more points wherein the process differs from 

 mitosis as it occurs in metazoan cells. When the centrosome 

 occurs within the nuclear wall, the entire process of division may 

 be accomplished without the disappearance of the nuclear 

 membrane. Frequently the chromatin is present as one or more 

 large bodies which are termed karyosomes. These constrict and 

 divide without chromosome formation, in a manner somewhat 

 resembling amitosis. The term promitosis has been applied to 

 this primitive type of indirect nuclear division to distinguish it 

 from the more elaborate mitosis. 



In some flagellates and rhizopods the chromatin is not con- 

 fined to a nucleus but is spread more or less uniformly throughout 

 the cytoplasm. Such distributed chromatin granules have been 

 observed by some investigators to form minute secondary nuclei 

 (as in Actinosphaerium) which become the nuclei of conjugating 

 gametes. 



Because of the important role that chromosomes have assumed 

 in discussions of heredity in the Metazoa, much interest has 

 centered around a discussion of the nature of chromatin masses 

 in Protozoa. Some investigators maintain that in many species 

 of Protozoa definite chromosomes appear in specifically character- 

 istic numbers. 



Many Protozoa have a definite life cycle more intricate than 

 a simple succession of fission states. Frequently, this cycle 

 involves two distinct types of adult individuals: sporonts, which 

 give rise to gametes, and schizonts, which give rise to asexual 

 individuals. There may thus be an alternation of generations 

 involving two distinct types of developmental cycle: a cycle 

 of sporogony, during which gamete formation and fertilization 

 occurs, and one of schizogony, during which no sexual process 

 is involved. These alternating cycles in the free-living forms are 

 correlated with environmental conditions, especially seasonal 

 changes, while in the parasitic species thay are frequently corre- 

 lated with change of host. In the malarial organisms (Fig. 29), 

 for example, schizogony continues during development of the 

 organism in the vertebrate host and sporogony involving the 

 fertilization of gametes is restricted to the sojourn of the parasite 

 in the body of the mosquito. 



