THE SPOROZOA 169 



as baby, child, youth and adult, used in common language 

 to designate the different stages of growth of human beings. 

 When two gametocytes become associated in a single cyst they 

 do not lose their identity ; they are only two grown-up parent 

 forms which are keeping company while the reproductive cells 

 that will give rise to a new generation are being formed and 

 matured. The production of gametes from each of the two 

 parent forms or gametocytes is analogous to the production 

 of ova and spermatozoa from multicellular animals, the differ- 

 ence being that in our parasitic protozoon the greater part of 

 the parent body is used up in the formation of the reproductive 

 cells. What remains is simply a granular residuum, serving 

 at the most as nutritive material for the developing progeny. 

 The conjugation of the gametes is clearly an analogous process 

 to the fertilisation of the ovum by the spermatozoon in higher 

 animals. But in Monocystis there is no visible differentiation 

 of sex ; the two conjugating gametes are so much alike that 

 it is impossible to say that one is male and the other female. 

 The parasite is therefore called isogamous.* The zygote 

 resulting from conjugation is clearly the equivalent of a 

 fertilised ovum which as a result of developmental processes 

 will grow into a new organism like its parent. It is at this 

 stage that the special features of sporozoon development come 

 into prominence. The zygote does not grow at once and 

 directly into the adult form. It begins by surrounding itself 

 with an impervious envelope, this being an adaptive structure 

 serving as a protection against destructive influences during 

 its transference from the old host to a new. Moreover the 

 zygote multiplies itself asexually within its protective coat. 

 The eight sporozoites formed by division of the sporoplasm 

 represent a new cell generation, whose obvious utility is to 



* Since this chapter was written R. Hofmann (Ueber Fortpflanzungser- 

 scheinungen von Monocystideen des Lumbricns agricola^ Archiv f. Pro- 

 tistL-nkunde XIII. , December 1908) has described anisogamy in a species 

 of Monocystis infesting one of the common earthworms. Of the two 

 associated gametocytes one produces gametes of the ordinary form ; the 

 ether gives rise to gametes shaped like Indian clubs, the nucleus being 

 situated at the junction of the handle with the body of the club. The act' 

 of conjugation was not observed, but it is presumed that the indian-club 

 shaped forms are male or microgametes and that each conjugates with an 

 oval female form or macrogamete. Thus, in one species of Monocystis, at 

 least, the gametes are of two kinds, but it appears that other species are 

 isogamous as described above. 



