1919 ] McCuUoch: Life Cycle of Crithidia and Trypanosoma 163 



tions of the "crop'' of Euryopthalmus convivus, namely, that which 

 leads to the formation of the somatella (pi. 3, figs. 28-32). These 

 spherical crithidias have certain characteristics in common with the 

 tailed and tailless spheres described in the life cycle of Trypanosoma 

 lewisi by Minchin and Thomson (1915). In a general way they re- 

 semble also the somatellas in the life cycle of some of the Polymastigina 

 described by Kofoid and Swezy (1915). Beginning with the earliest 

 stages of this type of multiple fission a series of flagellates can be ar- 

 ranged which parallels the successive stages of multiple fission of T. 

 lewisi (cf. Minchin and Thomson, 1915, pi. 37). At the beginning 

 of this series such flagellates as shown in figures 24 and 25 of plate 3 

 are to be found. In figure 24 the elongate flagellate is beginning to 

 round up. The attenuate ends are being drawn up to the central part 

 of the body and the flagellum has become intracellular throughout its 

 length. A somewhat different formation of the sphere is shown in 

 figure 25, plate 3. The long, attenuate ends are being wrapped about 

 the body and the flagellum is likewise entirely intracellular in this 

 flagellate. Other important features to be observed in connection 

 with this flagellate are the first indications of nuclear division in the 

 spherical formation. The centriole or centrosome within the nucleus 

 of the rounding-up flagellate is initiating the division of the organ- 

 elles. It has divided into two daughter-centrosomes, which are still 

 connected by a centrodesmose, and with each daughter-centrosome there 

 is present a varying amount of chromatin material. No chromosomes 

 are present at this stage of the division, and their presence at any 

 period throughout the process in Crithidia euryophthalmi has not been 

 established. Indications of chromosomes in the ordinary binary fission 

 have been observed, but no definite number has been determined. The 

 nuclear membrane has begun to constrict on either side of the centro- 

 desmose. The entire division of the chromatin material takes place 

 within the nuclear membrane, and the process is apparently a primi- 

 tive type of promitosis. The centrosome within the karyosome is not 

 always the first to divide ; on the contrary, in many instances the first 

 indication of the division of the organelles is shown by the blepharo- 

 plast or extranuclear centrosome. When the blepharoplast divides 

 the division of the parabasal body follows immediately. A repetition 

 of the division of the several organelles occurs and the spheres finally 

 break up into a number of merozoites, or daughter individuals. 



The spherical formation is completed in figure 26, plate 3. A 

 flagellum is protruding beyond the surface of the sphere. Internally 



