PROTOZOA 131 



are called gametes and there are the male or micro gametes, and the 

 female or macro gametes. The cells which produce the gametes are 

 called gametocytes. The product of the union is called a copula or 

 zygote. If the uniting cells be equal in size the copulation is isogamous; 

 if they be unequal, the copulation is said to be anisogamous. Aniso- 

 gamous copulation, the union of two unequal cells, is most typically 

 seen in the fertilization of a large macrogamete by a small microgamete. 

 Copulation is the most common fertilizing process among the patho- 

 genic protozoa. Conjugation, the second method of fertilization, only 

 occurs among the ciliata. In it, two adult individuals place themselves 

 in apposition. The nucleus of each cell first reduces and then divides 

 into two halves, one male, the other female. Each organism retains 

 its female half nucleus, while an exchange of the male half nuclei is 

 effected. Processes of self-fertilization, such as autogamy and partheno- 

 genesis, are included under the third heading. In autogamy the nucleus 

 of a single cell divides into two parts. Each of these may undergo 

 further division, during which the chromosomes are reduced or there 

 may be a simple extrusion of a portion of the chromatin. The two 

 resulting, reduced nuclei then unite, in the same cell, to form a new 

 nucleus. Parthenogenesis is the development of new individuals from a 

 female cell without a preceding fertilization; this process possibly occurs 

 in many protozoa, and through it perhaps may be explained the reap- 

 pearance of malaria in patients who once suffered from that disease 

 and were thought to have recovered. 



The LIFE CYCLE of a protozoon consists of the changes through 

 which it passes in the period intervening between each fertilizing act. 

 In many of the pathogenic protozoa, an alternation of generations 

 occurs; that is, cycles of development in which an asexual method of re- 

 production occurs, alternate with cycles of development in which re- 

 production is effected by sexual methods. The developmental cycles 

 are commonly punctuated by binary or multiple division, by encyst- 

 ment, and by transference to a second host as a necessary factor for the 

 completion of the life cycle. An alternation of generations occurs 

 in the life cycle of one of the most important of the pathogenic protozoa, 

 the parasite which produces malaria (Fig. 189). While it is in the body 

 of its mammalian host, man, it multiplies through multiple fission or 

 schizogony; the sexual, or propagative phase of its development 

 occurs within the body of its invertebrate host, a mosquito. The 



