CHAPTER XXXIII. MONOCYSTIS AND COCCIDITJM, EX- 

 AMPLES OF SPOROZOA (PARASITIC PROTOZOA). 



1. Structure of Monocystis. Various stages in the 

 life-history of this organism are almost invariably seen 

 when a small piece of a seminal vesicle of the earthworm 

 is cut off, slightly spread out on a slide, and examined with 

 the microscope. When the worm is opened, brownish or 

 black specks are usually seen on the surface of the seminal 

 vesicle. These are produced by the reproductive cysts of 

 Monocystis, and in them under the microscope are seen 

 groups of the spores. The mature organism before it 

 enters upon the reproductive stage can usually be found in 

 a preparation, and is a single cell of somewhat elongated 

 cylindrical form, limited externally by a smooth cuticle 

 (fig. 205, c). The interior substance shows a distinction into 

 external ectoplasm and more granular, somewhat opaque 

 endoplasm. The deepest part of the ectoplasm forms a 

 layer of fine contractile fibres, which have been called the 

 myocyte fibrillae. Within the endoplasm is the nucleus, 

 which is limited by a delicate membrane, and contains a 

 clear liquid in which are suspended denser bodies called 

 karyosomes. 



2. Formation of the Gametes. The Monocystis in the 

 stage we are considering is free in the contents of the 

 seminal vesicle, and moves by contractions of the ecto- 

 plasm. The next stage is the commencement of the process 

 of reproduction, and the organism in this stage is therefore 

 called the gametocyte, the cell which forms the gametes. 

 Two gametocytes come into close contact with each other, 

 and surround themselves with a spherical cyst, the gameto- 

 cyst, in which two layers can be distinguished, the epicyst 

 and endocyst (fig. 205, D). No fusion or conjugation takes 

 place between the gametocytes, but each proceeds to divide 

 independently by mitosis into a number of small cells, 



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