REPRODUCTION AND THE LIFE CYCLE 197 



passage of the eggs down the oviduct, develop in the yolk of the egg, 

 and heconie disseminated throughout the embrvonic cells, reproducing 

 the while, and finally lodging in the salivary glands of nymphs and 

 images. Similarly, with ticks of the genus Argas, which are known to 

 transmit spirochetes of different species infecting birds and fowls, 

 Levaditi has shown that the spirochete of relapsing fever or spirillosis 

 in chicks penetrates the ova of Argas mi)iiatu.f, and in this way infects 

 the young chickens. Relapsing fever in man due to Spirocheta duttoni 

 is conveyed by ticks of the genus ornithodorus, in the eggs of which 

 Carter ('06) and others have shown that the ova are frecpiently the 

 seat of multiplication of the parasites derived from the infected parent. 



Fig. 83 



x-^ / 





<- 





■"■f 





W^ 





iipii'l:**^"^-'- /- 



Section (jf lung infesteil by Treponema ijallidum; congenital sypiiilis. X 800. 



A final stage in the development of this means of transmission is sug- 

 gested by Ward ('OS), in connection with the })arasites of the intestine 

 of the housefly, which, no longer drawing blood, transmit the para- 

 sites from generation to generation only through tiie embryos. This 

 suggestion, however, loses weight from the fact made out by Patton 

 COS) that direct infection follows ingestion of encysted forms of the 

 intestinal parasites. 



\Vitli ni;in and niaininals transmission i)y inheiilance is nuicli more 

 difliciih, if for none but mechanical reasons. The |)arasites must 

 |)<'iicl rale llic j)lacciita and tlic solid tissue of (lie nnibilieal coi'cb and 

 it is conceivable tli;it only minute and highly motile forms can do .so. 

 It is a well-established fact, however, that certain kinds of parasites 



