REPRODUCTION AND THE LIFE CYCLE 197 



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

 and become disseminated throughout the embryonic 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 Arc/as miniatus, and in this way infects 

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

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

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

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



FIG. 83 



V (' -4- ' VV T 



' 



Section of lung infested by Treponema pallidum; congenital syphilis. X 800. 



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

 gested by Ward ('08), in connection with the parasites of the intestine 

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

 sites from generation to generation only through the embryos. This 

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

 ('08) that direct infection follows ingestion of encysted forms of the 

 intestinal parasites. 



With man and mammals transmission by inheritance is much more 

 difficult, if for none but mechanical reasons. The parasites must 

 penetrate the placenta and the solid tissue of the umbilical cord, and 

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

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



