1907.] NATURAL SCIENCES OF I'HILADELPIII A. 275 



The mature cysts arc about 15 microns in diameter and they are 

 invariably rounded, unless, as occasionally happens, they are distorted 

 by contact with adjacent cysts. Each cyst contains at least 30 spores. 

 As the spores are closely crowded together, I have not been able to 

 make out the exact number in any one cyst or to determine whether 

 the number is constant for all of the cysts. The young spore has 

 granular cytoplasm and a single, deeply staining nucleus (fig. 7, a). 

 It resembles somewhat the stage of the parasite shown in fig. 1, a; 

 but as it is considerably smaller and is always enclosed in a cyst, the 

 two stages are easily distinguished. In their later history the majority 

 of the spores follow one of two clearly marked modes of development, 

 which, for convenience in description, will be designated as Type I 

 and Type II. 



The first change that takes place in the spores that develop according 

 to Type I is the appearance of a clear area around the nucleus which 

 occupies the centre of the spores (fig. 7, h). Owing to the small size of 

 the spores it is impossible to determine whether the nucleus suddenly 

 becomes vesicular at this time or whether the clear area is a fluid vacu- 

 ole. Judging from the changes that take place in later stages of 

 development, I am inclined to the opinion that the character of the 

 nucleus does not change at this time and that the clear area around the 

 nucleus is a vacuole. Soon after the formation of the vacuole the 

 nucleus divides (fig. 7, c) and, as the spore elongates, the two nuclei 

 separate and move to opposite ends of the vacuole which increases 

 considerably in size (figs. 8, 9) ; subsequently the vacuole itself divides 

 and each part comes to surround one of the nuclei (fig. 10). At the 

 next stage the vacuoles, with the nuclei, are found at the ends of the 

 spore which has now attained its final shape (fig. 11). The nuclei 

 appear as large and stain as deeply at the stage of fig. 11 as they do in 

 the young parasite; but in a slightly older spore they are considerably 

 smaller and stain much less intensely (fig. 12). At a later period all 

 traces of the nuclei are lost and the vacuoles appear perfectly clear and 

 transparent (figs. 13, 14). 



During all stages in the development of the parasite up to that of 

 fig. 13, the cytoplasm appears granular and stains very faintly. When 

 the nuclei disappear at the stage of fig. 13, the cytoplasm in contact 

 with the vacuoles shows a strong affinity for the iron-haematoxylin, 

 although the rest of the spore stains as faintly as before. In a slightly 

 older stage the central portion of the cytoplasm stains as intensely as 

 the end regions, and the entire spore, excepting the vacuoles, appears 

 uniformly black (fig. 14). Somewhat later the vacuoles begin to 



