1919 ] McCulloch: Life Cycle of Crithidia and Trypanosoma 175 



into the pyloric expansion. Considering the attached crithidias the 

 most anterior sections of the pyloric expansion are almost identical 

 with the sections from the posterior part of the mid-stomach. The 

 lumen of the narrow constriction separating the mid-stomach from 

 the pyloric expansion is frequently almost blocked with the bodies of 

 the flagellates extending out into the passageway between these two 

 enlargements of the digestive tract. 



The structure of these haptomonads (pi. 6, figs. 93-96) is also 

 relatively uniform. The majority of the crithidias show the vesicular 

 type of nucleus, with a central karyosome and a nuclear membrane 

 containing an inner lining of chromatin material. In figure 94 the 

 nuclear membrane is slightly distorted in appearance and the karyo- 

 some is excentric, being posterior. In figure 96 the nuclear membrane 

 is not distinct and the enlarged mass of chromatin material is in the 

 form of two granules. These haptomonads are usually characterized 

 by their attenuate anterior and posterior ends. The posterior ends of 

 these crithidias more than of any others show relatively extreme 

 attenuation. 



Beginning in a region just posterior to the mid-stomach end of 

 the pyloric expansion a series of transition haptomonads is found 

 (pi. 6, figs. 97-106), lining the middle part of this division of the 

 digestive tract, these transition haptomonads, which vary consider- 

 ably in size. Figure 97 shows a broad, stout form. Such forms undergo 

 binary fission, producing two smaller, more slender individuals. Pos- 

 sibly figures 100 and 101 are the products of such a division. The 

 binaiw fission of the broad, stout forms rapidly increases the number 

 of haptomonads which make up the dense and compact layer of attached 

 crithidias. The nuclear structure of these forms shows compara- 

 tively little variation. They have the vesicular nucleus with the small 

 central karyosome and chromatin-encrusted nuclear membrane. 



At the posterior portion of the pyloric expansion are found the 

 normal rectal forms (pi. 6, figs. 107-124) which are common to the 

 life cycles of so many of these flagellates. One of the interesting 

 things observed in connection with these forms is that the wall of the 

 P3'loric expansion becomes exceedingly thin. There are few indica- 

 tions left of the epithelial cells lining this part of the mid-gut. Sec- 

 tions of this portion of the tract previous to any infection by the 

 flagellate are not at hand, unfortunately, and consecpiently it is diffi- 

 cult to estimate the total amount of destruction incurred. The pyloric 

 expansion of an infected adult bug, however, is extremely weak and 



