THE RAT-FLEA, CERATOPHYLLUS FASCIATUS BOSC. 451 



the contents of the body-cavity through the cut end of the trunk. 

 The salivary glands sometimes come out as soon as the flea is 

 decapitated, without any such pressure, and it is easy to get 

 them on to a cover-slip and fix them. 



Almost the only point in which the larval glands (PI. 27, A) 

 resemble those of the adult is in the characteristic structure of 

 the duct, which can be recognised immediately. Passing back 

 along the duct (d.), we come to a thin- walled dilated sac or 

 reservoir (r.), quite absent in the adult. Behind the duct a 

 tubule begins, composed of lightly staining glandular cells. After 

 a short course this tubule becomes continuous with the gland 

 proper, which is composed of darkly staining glandular cells, and 

 branches out into three lobes or diverticula, two of which run 

 forward (l.a. 1 , l.a. 2 ) and one backward (l.p.) alongside of the 

 digestive tract. All this arrangement of duct, reservoir, and 

 gland is, of course, duplicated on each side of the body, right 

 and left. 



Accompanying the larval salivary gland are two elongated 

 pads or cushions of fat-body, which are very difficult to separate 

 from the gland without damaging the glandular lobes. In the 

 hinder of these pads of fat I found in many fleas a body which 

 looked exceedingly like a parasitic cyst, for which I mistook it at 

 first. Specimens mounted whole showed the " cyst " to be com- 

 posed of large cells in the interior, showing a. tendency in the 

 more advanced specimens to arrangement in longitudinal rows, 

 and enveloped by a layer of flat epithelium at the surface. At 

 its hinder end the " cyst " is prolonged into a delicate cord of cells 

 which could be traced in some specimens a long way back. 

 Further investigation showed, however, that when this " cyst " 

 was present on one side of the body it was also present on the 

 other side in exactly the same degree of development ; and further, 

 that when the "cysts" were absent in the fat-body on the level 

 of the salivary glands, they were to be found in other pads of fat- 

 body situated farther back, on the level of the intestine right 

 and left. Hence it was obvious that the supposed parasitic cysts 

 were simply the genital rudiments, situated farther forward in 

 the larvae of one sex than in the other. Whether it is the male, 

 or the female, in which they are situated farther forward, I 

 cannot say. 



The striking differences between the larval and adult flea in 



