REPRODUCTION 



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two flagella they are capable of active motion. The macrogamete {XIc 

 and XIII) is a large cell, rich in protoplasm and non-motile. In fertili- 

 zation a microgamete fuses with a macrogamete, giving rise to a zygote 

 which secretes a cyst about itself and may now be called an oocyst. 

 Within the oocyst four resistant spores arise by division of the nucleus 

 and cytoplasm and the secretion of resistant coats about the small 

 cells. Within each spore the cell divides again to form sporozoites. 



In the gregarines, of which Monocystis may be taken as an example, 

 fertilization occurs between nearly similar gametes. Two full grown 

 nutritive individuals come together without fertilization and secrete 

 a common cyst wall. Within each cell the nucleus and cytoplasm divide 

 repeatedly to form a large number of minute cells. The old cell mem- 

 branes now break down and the small cells fuse two by two to form 



Fig. 136. — Reproduction in Eudorino elegans Ehrenberg. A, adult colony X475; B, 

 daughter colony produced by division of one of the cells of such a colony as in ^4., x730; 

 C-E, development of spermatozoa from a mother cell; F, spermatozoa. (From West after 

 Goebel.) 



zygotes. These become spores by secreting resistant coats. Within the 

 spores the cell divides to form 8 minute sickle-shaped sporozoites. The 

 diagram (Fig. 137) shows a scheme of spore formation in gregarines. 



In the examples of protozoan sexual reproduction cited there is 

 complete fusion of one gamete with the other, whether the union is 

 between like or unlike gametes; but in some protozoan species, for ex- 

 ample, in certain Infusoria (Ciliata) which possess a macro- and a micro- 

 nucleus, the gametes do not fuse. There is in such cases merely an 

 exchange of portions of the micronuclei and perhaps of some cytoplasm. 

 This union for the exchange of nuclei is called conjugation. The com- 

 plete fusion of protozoan cells has sometimes been given another name 

 (copulation) which, however, is used in a totally different sense with re- 

 spect to the metazoa, so that it seems wise here to refer to such complete 



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