286 MONOCYSTIS CHAP. 



T 1 7 mm. in diameter, can usually be seen if a small piece 

 of a sperm-sac is examined under the microscope. When 

 ripe each cyst contains a large number of small spindle- 

 shaped or boat-shaped spores (E). Before encystment, 

 Monocystis has a somewhat elongated form, and is 

 capable of slow movements amongst the cells of the 

 sperm-sacs, contractions of the organism 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 succes- 

 sive divisions (C), the ultimate products of which, each 

 surrounded by protoplasm, constitute small gametes, part 

 of the protoplasm of the gametocytes remaining un- 

 divided and serving as nutrient material for the develop- 

 ing spores. 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 charac- 

 teristic spores already referred to, each of which is about 

 TU-rV mm - in length. 



The contents of each spore then undergo division (F) 



