286 MONOCYSTIS CHAP. 



a somewhat elongated form, and is capable of slow movements 

 amongst the cells of the sperm-sacs, contractions of the or- 

 ganism as a whole, or constrictions in various parts, causing 

 its shape to vary considerably at different times (A). There is 

 a clear cortical layer covered by a cuticle (p. 264), and a 

 very granular medullary portion, enclosing the spherical 

 nucleus (nu) in which one or more nucleolus-like bodies can 

 be distinguished. No pseudopods or cilia are formed, and, 

 as in Opalina, there is no mouth, gullet, or contractile 

 vacuole, fluid nutriment being absorbed from the cells of 

 the host. During this stage the organism feeds and grows, 

 and is spoken of as a trophozoite. 



After a time two trophozoites become applied together, 

 without fusing, and a common resistant cyst is formed around 

 them : each is now known as a gametocyte (B). The nucleus 

 of each gametocyte then undergoes successive divisions (c), 

 the ultimate products of which, each surrounded by proto- 

 plasm, constitute small gametes, all of which are similar to 

 one another. 1 These gametes then conjugate in pairs (D), 

 probably those derived from one gametocyte uniting with 

 those derived from the other. The result is the formation of 

 numerous zygotes, each of which becomes spindle-shaped and 

 surrounded by a tough cell-wall, so that the cyst (E) now 

 contains a number of the characteristic spores already 

 referred to, each of which is about -^ ^ mm. in 

 length. 



The contents of each spore then undergo division (F), 

 forming eight elongated and nucleated bodies or sporozoites, 

 which become set free by the bursting of the enclos- 

 ing membrane : apparently, however, they must enter a 



1 During this process part of the protoplasm of the gametocytes 

 remains undivided and serves as nutrient material for the developing 

 spores. 



