POLYMORPHISM AND LIFE-CYCLES 173 



chapter, the macramcebae produced are fewer than the micramoebse, 

 showing that the tendency to division is more restricted in the 

 former case than in the latter. Again, in the development of Centro- 

 pyxis, as described by Schaudinn (131), formation of gametes is 

 initiated by a process of multiple fission combined with formation 

 of secondary nuclei from chromidia, as in Arcdla, and in this way 

 a number of amcebulse are produced. The amcebulse from one 

 Centropyxis remain undivided, as macramcebse, while those pro- 

 duced from another adult divide each into four micramcebae ; 

 syngamy takes place later between a micramceba and a macramceba, 

 after each has secreted for itself a shell. 



When the inhibition of the gamete-forming divisions is quite 

 complete in one sex, the result is the most pronounced type of 

 anisogamy occurring in Protozoa ; and, conversely, it may be said 

 that all cases of extreme anisogamy in Protozoa are of this type. 

 In Metazoa the disproportion in the size of the gametes is mainly 

 due to the relatively enormous growth of the gametocyte, partly 

 also to the inequality of the four cells produced by the reducing 

 divisions, in the female sex. In Protozoa with extreme differen- 

 tiation of gametes, on the other hand, such as the Coccidia and 

 Hsemosporidia, the gametocytes do not differ greatly, sometimes 

 not at all, in size, though the female gametocyte may contain 

 more reserve food - material, and consequently less protoplasm. 

 The disproportion of the gametes is due almost entirely to the fact 

 that in the female sex the gametocyte does not divide, but becomes 

 a single macrogamete, while the male gametocyte sporulates to 

 produce a larger or smaller number of microgametes. 



Very instructive in this respect is the comparison of the formation 

 of the gametes in the gregarines (p. 331) and the coccidia (p. 346) 

 respectively, two groups of Protozoa which are certainly closely 

 allied to one another. In such a form as Coccidium (Fig. 152), the 

 gametocytes remain separate one from the other, and the male 

 gametocyte forms numerous minute microgametes which swarm 

 away ; the female gametocyte, on the other hand, becomes a macro- 

 gamete after going through a process of reduction, and is fertilized 

 by a single microgamete. In gregarines, however, the gametocytes 

 associate in couples, either before or after attaining their full size, 

 and become surrounded by a common cyst, within which each 

 gametocyte sporulates to produce a large number of small gametes. 

 The gametes of gregarines can be arranged in a series, showing 

 marked anisogamy at one end, complete isogamy at the other. 

 Thus in PterocepTialus (Fig. 79, A, B) the gametes are very unequal 

 in size, and the microgametes are motile, the macrogametes not so. 

 In Stylorhynclms the gametes of opposite sexes are equal in size, 

 but in one sex the gametes are motile, in the other not (Fig. 79, 



