1919] McCulloch: Life Cycle of Crithidia and Trypanosoma 173 



scarcely begun to develop. As the developing forms and the fully 

 matured forms enter the pyloric expansion, they become part of the 

 permanent crithidia! infection therein. The content of the crithidial 

 infection of this region varies from time to time. Nectomonads 

 may predominate or all nectomonads may become haptomonads, or 

 attached forms. Usually the nectomonads and haptomonads are both 

 present in large numbers. Mature nectomonads in the mid-stomach 

 are usually the elongate, slender crithidias (pi. 4, figs. 73-79). In 

 figure 74 the body size is approximately 25^ in length and 1.7,u in 

 width. Figures 75 to 78 are yet narrower, averaging about 1/x in 

 width. The nectomonads of the pyloric expansion are frequently 

 like figures 73 to 79, long, slender forms together with numerous 

 shorter and stouter crithidias (pi. 4, figs. 80-90). It is conceivable 

 that the elongate slender forms give rise to the shorter, stout forms. 

 The length of the body is decreased and the width is increased. Both 

 the anterior and the posterior ends of the body become less attenuate. 

 The structure of the nucleus of the elongate forms varies con- 

 siderably. In figures 73 and 80 there are distinct central karyosomes 

 with chromatin-encrusted membranes. In figure 74 the chromatin 

 is in five granules scattered within the nuclear membrane. The 

 chromatin is broken up into granules in figures 76 and 79. The break- 

 ing up of the chromatin material in these nectomonad forms doubtless 

 means the beginning of a degeneration, which will be discussed shortly. 

 The nuclear structures in figures 77 and 78 are unique. In figure 

 77 the nuclear membrane is rather densely encrusted with chromatin 

 and is elongate and irregular in outline. In figure 78 the nuclear 

 membrane is oval, chromatin-encrusted, with two masses of chromatin 

 at the anterior and posterior regions of the nucleus. In the series 

 of figures 81, 83 to 86 a karyosome of a variable size is found in 

 each and its location is not always central. In figure 82 a distinct 

 chromidial fragmentation of the chromatin has taken place, which is 

 another indication of degeneration. The short, stout forms (pi. 4, 

 figs. 87-90) are transition forms from the nectomonads to hapto- 

 monads. "While these flagellates are still free forms they resemble the 

 haptomonads of the pyloric expansion, which are attached in the mid- 

 region of this division. In the anterior part of the pyloric expansion 

 the haptomonads are relatively long, slender flagellates while posterior 

 to the middle portion they are still short and more pear-shaped. The 

 nuclear structure of these transition forms indicates no degeneration 

 as yet. They all have a central karyosome and a more or less encrusted 

 nuclear membrane. 



