504 University of California Puhlicati-ons in Zoology ["\'ol. 20 



A daily examination of the normal faeces, as they may be pressed 

 from the rectum of the rat, shows that there is no regular periodicity 

 in the formation of cysts (Brug, 1919, and Kessel, 1923a.). At times 

 as long an interval as three weeks may elapse between the appearance 

 of cysts in the faeces, while at other times they may appear daily for 

 three consecutive days. 



It is possible to detect the cysts in normal saline solution by their 

 light gray color and bj^ the inner darker layer of the peripheral wall 

 which surrounds the cyst. It is, however, extremely difficult to dis- 

 tinguish the nuclei, and differentiation of species should not be based 

 on this type of superficial examination alone. In this preparation the 

 glycogen mass appears as a large fluid-filled vacuole. 



In the iodine-eosin stain the cysts assume a greenish-yellow color, 

 presenting a shade which is about midway between the pale yellowish 

 green cysts of Endamoeba dysenteriae, and the more yellow cysts of 

 Endamoeba coli. In this preparation the nuclei stain definitely, and 

 the type of karyosome often may be clearly demonstrated. The 

 glycogen mas.ses which are present in a certain number of the cj'sts 

 are always stained dark brown by the iodine. 



Shapf. — The majority of the cysts are spherical or ellipsoidal in 

 shape, in optical section appearing round (pi. 45, fig. 23), perfectly 

 elliptical (pi. 45, fig. 20), or presenting varying degrees of elliptical 

 outline (pi. 43, fig. 7; pi. 44, fig. 10). No extremely irregular cysts, 

 like those often encountered in C. decumani (pi. 47, fig. 49), have ever 

 been observed by us in C. mitris. 



Cyst wall. — A thin wall surrounds the cyst. In the iron-haema- 

 toxylin stain this appears quite transparent and can be defined only 

 by the inner and outer margins. In the iodine-eosin preparations, 

 however, it appears as a circle composed of three layers, in optical 

 section consisting of a thin outer line, a middle clear zone, and an 

 inner dark line. This resembles the cyst wall of C. lafleiiri (Kofoid 

 and Swezy, 1921). The thickness of the cyst wall is relatively con- 

 stant in all cysts, no matter what their dimensions may be. Measure- 

 ments show only a slight variation from 0.50;* in the newly-formed 

 cy.sts to 0.75/t in the larger and older cysts. 



Sizes and races of CouncUmania muris. — Wenyon and O'Connor 

 (1917) and Dobell and Jepps (1918) have shown that E. dysenteriae 

 and E. coli are divided into a number of distinct races, the determina- 

 tion being based on the size of the cysts. The five races of E. dysen- 

 teriae have mean diameters of 6.6, 8.3, 11.6, 13.3, and 15/i respectively. 



