302 



THE PROTOZOA 



published results, in rare instances, in striking contrast with the ease with 

 which the transmission is effected by the rat-flea. The rat-louse may be 

 regarded, therefore, as a host in which the trypanosome establishes itself 

 only exceptionally, and by no means' as the host to which it is best adapted. 

 Crithidial and other forms have been seen in the louse, but the intracellular 

 phase has not been observed, and it is probable that most of the forms de- 

 scribed from this host are degenerating forms maintaining a feeble and pre- 

 carious existence under adverse conditions, and destined to die off and dis- 

 appear sooner or later. 



The developmental cycle of Schizotrypanum cruzi in the bug Conorhinus 

 megistus has been described by Chagas,* and is briefly as follows (Fig. 132) : 



J 



FIG. 132. Phases of Schizotrypqnum cruzi in the bug Conorhinus megistus. 

 a, b, and c, Forms transitional from the ordinary trypanosomes to the rounded 

 forms ; d, clump of rounded forms ; e and /, change of rounded into crithidial 

 forms ; g and h, crithidial forms ; i, trypaniform type from the salivary 

 glands ; j, encapsuled form from the intestine. After Chagas (425). 



The trypanosomes taken up by the bug into its stomach change in about 

 six hours ; they lose their flagellum and contract into rounded, Leishmania- 

 like forms, which multiply actively by fission. After a time multiplication 

 ceases, and the rounded forms become pear-shaped, develop a flagellum at 

 the pointed end, and change into typical crithidial forms which pass on into 

 the intestine, and there multiply by fission. In this way the characteristic 

 condition of the infected bug is produced, with the intestine containing a 

 swarm of trypanomonad individuals multiplying actively. The final stage 

 in the insect is a small trypaniform type which is found in the body- cavity 

 and salivary glands, whence it doubtless passes into a vertebrate host again. 



* A critical summary and review of the memoir of Chagas is given by Minchin 

 in Nature, vol. Ixxxiv., pp. 142-144 (August 10, 1910), with three text-figures. 



