6oo 



Plague 



was begun within two days from the time of inoculation the animals 

 could be saved, even though symptoms of the disease were marked. 

 After the second day the treatment could be relied upon. The dose 

 necessary was 20 cc. of a serum having a potency of i : 10. If too 

 little serum was given, the course of the disease was regarded and the 

 animal impoved for a time, then siiffered a relapse, and died in 

 from thirteen to seventeen days. The serum also produced im- 

 munity, but of only ten to fourteen days' duration. Immunity 

 lasting three weeks was conferred by inoculating a monkey with an 

 agar-agar culture heated to 6o°C. If too large a dose of such a 

 culture was given, however, the animal was enfeebled and remained 

 susceptible. 



THE PLAGUE FLEAS 



Fleas were formerly classed as a suborder of the Diptera, or two-winged insects, 

 and because they had no wings, were known as Aphaniptera. At the present 

 time they constitute an order by themselves, the Siphonaptera. 



Fig. 238. — Various fleas, magnified about 30 diameters. The specimens are 

 treated with hot 20 per cent, caustic potash for a few minutes, dehydrated in 

 alcohol, cleared in xylol and mounted in balsam, a, Ceratophyllusfasciatus, cf ; 

 b, Ceratophyllus fasdatus, 9 ; c, Leptopsylla musculi, cf ; d, Leptopsylla musculi, 9 

 (Bacot, in Journal of Hygiene, "Plague Supplement in, i9r4"). 



Every flea undergoes a complete metamorphosis. It begins its life history as 

 a minute, oval pearly white egg measuring about 0.6 mm. in length, that falls 

 from the body of the female to the floor or ground. The eggs of fleas are not 

 cemented to the hairs like those of Uce, but drop to the ground where the larva 

 lives. More or less eggs are therefore always scattered about where dogs, cats, 

 rats, mice or other animals that harbor fleas are to be found, and more or less 

 larvae and pupae are likewise to be found in such places. In the course, of from 



