THE SPOROZOA 183 



granular endoplasm. Within the membrane is a fluid nuclear 

 sap in which float one or more karyosomes, held in place by a 

 delicate nuclear reticulurn. Each karyosome is usually a spherical 

 body of vacuolated structure, as described for Monocystis. Some- 

 times the karyosome is drawn out and band-like (Fig. 44) or beaded. 

 As a general rule, however, the nuclei of Gregarines exhibit a 

 monotonous uniformity of structure and appearance. 



The reproductive phase of the life-history is usually initiated, 

 as has been said, by the association of two or more sporonts, but 

 not infrequently a single Gregarine may become encysted by 

 itself, without pairing with another. In this case the sporont 

 breaks up into sporoblasts at once, a fact which may be expressed 

 in another way by saying that the gametes develop without 

 conjugation into spores, a proceeding which may be compared to 

 parthenogenesis. In such cases the spores are smaller than those 

 produced from zygotes. 



The cyst is a structureless membrane secreted by the associated 

 sporonts. Its function is to afford protection to the reproductive 

 phases, and when once it is fully formed, subsequent develop- 

 ment can proceed outside the body of the host. The cyst-wall 

 consists commonly of a gelatinous outer layer, often of considerable 

 thickness and showing concentric striations termed the epicyst, and 

 a tough inner membrane, the endocyst. Some interesting mechan- 

 isms are found which have as their object the facilitation of the 

 escape of the spores from the cyst. In the majority of cases 

 the cysts dehisce by rupture of their walls, brought about by 

 swelling of their contents, and in particular of the residual proto- 

 plasm, or by contraction and shrinkage of the cyst-wall, or by 

 both causes combined. In other cases the residual protoplasm, 

 after the sporoblasts are separated off from it, undergoes further 

 development to produce a special mechanism. In the families 

 Dactylophoridae and Slylorhynchidae, the residual protoplasm forms 

 a compact mass which becomes surrounded by a membrane and 

 gives rise to a structure termed a pseudocyst, which gradually 

 swells until the true cyst-wall is burst asunder. In several genera 

 of Gregarinidae ( = Clepsydrinidae auct.) the cysts are remarkable for 

 the possession of sporoducts (Figg. 28 and 42), long tubular out- 

 growths through which the spores can pass out to the exterior. 

 These ducts are also formed from the residual protoplasm, which 

 take up a peripheral situation within the cyst, surrounding the more 

 centrally placed spores. It then secretes a membrane immediately 

 internal to the cyst-wall, and also gives rise to a variable number 

 of tubes, usually six or eight, but sometimes only one, which at 

 their first formation run inwards from the periphery of the cyst, 

 but later become everted to form the sporoduct. 



The conjugation, so far as it has been observed, conforms to 



