1905.] NATURAL SCIENCES OF PHILADELPHIA, 159 



between the parasite and the host cell. It is often impossible, in the 

 case of those small closely applied forms, to get a sharp line of demarka- 

 tion between the sporozoan and the epithelium, the one blending with 

 the other by imperceptible degrees. 



However this may be, the juxtaposition of the parasite and the cell 

 is evidently of no great importance to the former. Ordinarily the 

 smaller, undeveloped stages are attached and the later stages free. 

 The reverse may, however, be observed and the matter is evidently one 

 largely of chance. 



By a uniform growth in all directions the trophozoite becomes 

 an egg or potato-shaped organism, reaching a length of around 20 

 microns. ^This, however, is to be noticed only in the free individuals 

 and where the spatial relations are favorable. More usually the 

 lumina of the tubules are so small and so closely crowded with the 

 parasites that the latter are constrained to assume a vermiform or 

 plate-like shape. Thus fig. 2, which is the longitudinal section of 

 an indivdiual with a circular cross-section, would 

 answer equally well for the cross-section of an 

 individual extending for some distance along the 

 tubule cells. 



Both these elongated or flattened individuals, 

 as well as the egg-shaped forms mentioned above, 

 may still retain the primitive character of being 

 naked masses of uniform cytoplasm with a vari- 

 ous number of solid nuclei. Generally, however, 

 the reproductive cycle is inaugurated while the 

 organism is still very small. The first indica- jrjg 2. 



tions of this are furnished by the nuclei. 

 These lose their spherical form and their solidity. They may become 

 either somewhat irregular masses or else rings. The elements are so 

 minute that the determination is difficult, but from what takes place 

 later these early phenomena are apparently the expression of the 

 l^reaking up of the nucleus into extremely small chromosomes. 



Simultaneously there arises around each nucleus a vacuole, which, 

 in its turn, can often be seen to be surrounded by a condensed belt of 

 cytoplasm. These are the first steps in the breaking up of the tropho- 

 zoite into separate elements, and from this point on development may 

 progress along either one of two lines. In the one case the .result is 

 the production of what I shall call the "round bodies." In the other 

 the process is clearly spore-formation. I have not yet succeeded in 

 satisfactorily differentiating these two developmental courses in their 



