154 University of California Publications in Zoology [VOL. 19 



may be carried immediately to the pyloric expansion through the mid- 

 stomach by the current of food, or they may undergo a process of 

 multiple fission extracellularly or intracellularly in the "crop." The 

 process of multiple fission may be similar to that of the spheres of 

 Trypanosoma leivisi (Minchin and Thomson, 1915), which have certain 

 characteristics in common with the somatella in the multiple fission of 

 some of the Polymastigina (trichomonad flagellates) described by 

 Kofoid and Swezy (1915). In addition to this type of multiple fission 

 there is a second and entirely different process of multiple fission, the 

 internal or endogenous budding (pi. 2, figs. 11-23). These two pro- 

 cesses of multiple fission have been described briefly in a preliminary 

 communication (McCulloch, 1917). Endogenous budding has been 

 emphasized in this paper and described in detail because of its interest 

 and importance. 



EXTRACELULAR CRITHIDIAS 



Oval spores. The initial infective spores (pi. 2, fig. 1) are ovoid 

 and stain deeply. They are found in small numbers in the "crop," 

 and present several distinguishing marks which serve as a means of 

 identification. They average 1.7/>t in width and about 3.4/A in length. 

 The nucleus stains diffusely and forms a solid mass of chromatin at 

 the extreme posterior end of the body. The parabasal body lies within 

 the anterior half of the spore, about equally distant from the nucleus 

 and forward end. One end of this bar-shaped structure, or parabasal 

 body, lies close to the thick periplast. On all the other sides of this 

 organelle there is the characteristic light area, which quickly destains 

 in iron-haematoxylin preparations. Careful focusing has revealed a 

 faint nuclear rhizoplast passing from the nucleus toward the region 

 of the parabasal body. 



Developing crithidias. When the initial infective spores begin 

 to develop in the "crop" a change occurs in their shape and staining 

 capacity (pi. 2, fig. 9). The forward outgrowth of the flagellum 

 assists in the elongation of the anterior end and the formation of the 

 undulating membrane (pi. 2, figs. 6-9). The posterior region elon- 

 gates less rapidly, but in time it frequently attains an even greater 

 length than the anterior end (pi. 2, fig. 9). The length of the free 

 flagellated crithidias which result from the developing forms varies 

 greatly at all times regardless of their location in the digestive tract. 

 In figure 9, plate 2, the crithidial flagellate has reached the extreme 



