174 



THE PROTOZOA 



C, D). In M&nocystis (Fig. 79, O L) the gametes differ slightly in 

 size in the two sexes, but have no organs of locomotion in either 

 case. In Urospora (Fig. 79, E, F) the gametes are not appreciably 

 different in size, but in those of one sex the nuclei are slightly 

 smaller than in those of the other. Finally, in Gregarina, Diplodina, 

 and many other genera, no difference whatever is perceptible 

 between the two gametes that perform syngamy. In those gre- 

 garines which have dimorphic gametes, syngamy is always between 

 two dissimilar individuals of distinct parentage, and it may be 

 inferred, therefore, that in all cases alike the gametes that unite are 

 derived from distinct gametocytes. 



FIG. 79. Gametes of different species of gregarines. A, Male, B, female, gamete 

 of Pterocephalus (Nina) gracilia. G and D, Stylorhynchua longicoUis : C, male 

 gamete ; D, male gamete attaching itself to a female. E, Male, F, female, 

 gamete of Urospora lagidis, showing differences in the size of the nuclei. 

 G L, Monocystis sp. : 0, male gamete ; H, female ; /, union of the two 

 gametes, the nuclei still separate ; J, the two nuclei fusing ; K, the zygote 

 becoming elongated ; L, the zygote has taken the form of the spore, and in 

 the synkaryo.n a centrosome has appeared, preparatory to division. A and 

 B after Leger and Dubosoq ; C and D after Leger ; E L after Brasil. 



From a comparison of the life-cycles of the Coccidia and the 

 Gregarines respectively (see p. 354, infra), it is highly probable that 

 in the common ancestor of the two- groups the gametocytes were 

 separate, as in Coccidium, and each produced numerous gametes, 

 as in Gregarines. Since the gametes had to find each other, by a 

 process of adaptation, those of one sex became smaller and more 

 motile (microgametes), while those of the other sex were more bulky 

 and inert (macrogametes). 



In the course of their evolution from this primitive ancestral 

 type, the Coccidia, with some exceptions presently to be noted, 

 retained the habit of the gametocytes, remaining separate, and the 

 specialization of the gametes became greatly increased, as an adap- 



