470 



FAMILY: TRYPANOSOMID^E 



physiological one, in that they cease to bring about infection if injected 

 into rats (Fig. 200, 1-4). They appear to become more rigid in character, 

 and possibly more violent in their movements. At about the end of this 

 period invasion of the lining cells of the stomach takes place (Fig. 20, 

 4-12). Actual penetration was observed by Noller (1912^), who saw a 



-^'c^ ^^f^ 



^p^y? 



Fig. 



199. — Diagram of Trypanosoma lewisi in the Blood of the Eat and 

 IN THE Flea. (After Wen yon, 1922.) 



A. Trypanosomes as seen in the rat at late iDhase of infection (X 1,500). 

 S. Trypanosomes in stomach of flea. 



B. Intracellular phase of development in stomach (X 1,500). R. Rectal phase. 



C. Attached flagellates in rectum : evolution of crithidia into metacyclic trvpanosome 



form ( X 1,500). 



D. Free metacyclic trypanosomes which bring about infection Mhen ingested bv rat 



(X 1,500). 



trypanosome enter the cell by its posterior end. Within the cell a vacuole 

 forms, in which the trypanosome may be seen to exhibit active movements. 

 It becomes doubled on itself, the two limbs of the U thus formed merging 

 into one another to form a pear-shaped body. The volume of this pear- 

 shaped body appears to be less than that of the trypanosome that entered 



