172 University of California Publications in Zoology [Vol.19 



(pi. 4, figs. 40-17) from l h (pi. 4, fig. 40) to 1.5/* (pi. 4, fig. 47) in 

 length and from 0.7 to 1/x in diameter. In comparing them with 

 the initial infective oval spores (pi. 2, figs. 1, 2) it is noticed that they 

 are smaller, stain less densely, and do not show a heavy periplast. The 

 nuclear structure also is unlike that of the initial spore forms, and the 

 location of the nucleus within the zooid adds another distinguishing 

 character. These zooids have always been found in the anterior por- 

 tion of the mid-stomach in the serial sections while the sections imme- 

 diately posterior contain developing crithidias (pi. 4, figs. 59-64). 

 Groups of elongating forms are frequently found (pi. 4, fig. 59). 

 With the elongation of the body of the zooids, especially of the ante- 

 rior end, the nucleus instead of being filled with chromatin is now 

 vesicular and contains a distinct karyosome. This group (pi. 4, 

 fig. 59) shows the parabasal bodies in close proximity to the nuclear 

 membranes. Judging from the conditions found within the great 

 majority of forms, figure 59 is probably an exceptional case in this 

 respect. The parabasal body normally moves anteriorly in the early 

 development of the zooid (pi. 6, fig. 40) before the flagellum and the 

 anterior end grow out (pi. 4, figs. 41-53). In the figures just noted 

 the flagellum is not yet visible. The nuclear rhizoplast, extending 

 from the nucleus forward to the region of the blepharoplast and para- 

 basal body, is found by focusing carefully. In figure 54 the flagellum 

 is growing out from the blepharoplast but there is no noticeable 

 lengthening of the anterior end of the body. In figures 55 to 58 the 

 flagellum and anterior end of each are lengthening simultaneously. 

 In these same figures the nuclei are like the nuclei of the zooids, being 

 completely filled with chromatin. Farther on posteriorly are found 

 crithidias such as are shown in figures 63 to 72. Beginning with 

 figure 65 there is also an elongation of the posterior end, which is equal 

 to that of the anterior. 



The majority of these developing crithidias have the vesicular 

 type of nucleus. Figures 65, 67, and 68 are exceptions, but no sig- 

 nificance can be attached to them since the position, relative thickness 

 of the body, or the technique, could explain these exceptions in this 

 region of the digestive tract. The study of the serial sections leads 

 us to think that the time necessary for the stomach crithidias to 

 reach the rectum is approximately the amount of time required for 

 the zooids to develop into mature flagellates. Not all of the develop- 

 ing zooids become mature in the mid-stomach. Under certain con- 

 ditions the food current probably carries many of the non-flagelated 

 stages or zooids back into the pyloric expansion before thej r have 



