264 



THE PROTOZOA 



subtilis (Fig. Ill) by Dobell (335). In this species the two gametes 

 appear perfectly similar to each other, and are not, in fact, distin- 

 guishable in any way from ordinary individuals of the species. Two 

 such individuals come together and unite by their anterior or flagellar 



extremities. In one 

 gamete the flagnllum is 

 lost, and the couple swims 

 about by means of the re- 

 maining one ; this is the 

 only difference between 

 the two gametes which 

 could be interpreted as 

 one of sex. While fusion 

 of the bodies is still 

 incomplete, the nucleus 

 of each gamete divides 

 by a simple type of 

 promitosis (p. 109). One 

 of each pair of sister- 

 nuclei thus produced is 

 a reduction - nucleus, 

 which degenerates ; the 

 other persists. The per- 

 sistent nucleus of each 

 gamete then divides a 

 second time, but into 

 two very unequal halves ; 

 the smaller nucleus in 

 each case degenerates as 

 a reduction -nucleus, 

 while the larger persists 

 as the pronucleus. The 

 bodies of the gametes are 

 now completely fused, 

 and the fusion of the 

 pronuclei follows. The 

 zygote may become en- 

 cysted at once, or may 

 continue to live a free 

 life. In the first case 

 the fusion of the pro- 

 nuclei takes place within the cyst, from which it is ultimately set 

 free as an ordinary individual which feeds and multiplies vegeta- 

 tively. In the second case the zygote becomes an ordinary free 

 individual at once, the interlude of encystment being omitted. 



Fio. 111. Life-cycle of Copromonas subtilis. A, 

 Ordinary adult form ; B, C, D, " vegetative " 

 reproduction by binary fission ; E J, stages of 

 reduction and syngamy : F , 0, H, reduction ; 

 /, J, fusion of the two pronuclei ; the /.ygote 

 (/) may develop into an ordinary free-swimming 

 individual, or (J) may retract its flagellum and 

 become encysted ; K, cyst ; L, liberation of an 

 adult form from the cyst. After Dobell (335). 



