168 H. M. WOODCOCK. 



cluster. This is an especially favourable position for them to become 

 attached, since the oesophageal epithelium has only lately shed its cuticle to 

 form the gelatinous sheath around the second quantity of blood, and its 

 cells are being actively regenerated prior to secreting a fresh one. The 

 Trypanosomes, therefore, are able to penetrate the delicate surface of this 

 layer, to which, indeed, as many as possible cling. With the increasing 

 scantiness of nutriment elsewhere more parasites are drawn into the imme- 

 diate neighbourhood, and these press in between those alreadj' attached 

 until finally, at the close of the second digestive period, an enormous mass 

 of parasites has accumulated at this spot, arranged in rows and layers, and 

 all in a resting condition. By this time the new cuticle has become 

 firm and chitinous, and when, at the next meal, the oesophageal invagination 

 is withdrawn out of the neck of the stomach, it leaves behind it the cuticle, 

 serving as a base of attachment for the mass of agglomerated Trypano- 

 somes. 



The next (the third) inflow of blood drives this mass before it, in the 

 form of a roUed-up ball, until it reaches the junction of the ileum and colon 

 (fig. 4, c), the narrowest point of the intestine. The wall is here very thin 

 and easily ruptured on distension. In this way most of the Trypanosomes 

 are enabled to pass through it, into the vascular lacunae around, whence 

 they are carried to the heart. From the heart, the Trypanosomes are borne 

 through the aorta into tiie sinus surrounding the pumping-organ of the 

 pharynx ; and between the latter and the phar^'ngo-oesophageal valve ' they 

 become at length arrested. The parasites continue to slowly multiply and 

 gradually collect again into agglomerated masses, which surround this 

 region of the pharynx and press on its walls, owing to the narrow throat of 

 the Insect. By the close of the third digestive period, these clumps of 

 Trypanosomes have broken through, and partly block up the cavity of the 

 pharynx. In the next biting act they are forcibly ejected thence into the 

 blood of the owl, as above described. 



Thus the parasites cannot leave the gnat until the fourth meal, including 

 that which effected their entry, or not until the third meal after infection 

 has taken place. Schaudinn found that the shortest time elapsing between 

 entrance and exit was seven or eight days ; this is the case when the Insects 

 are maintained at the optimum temperature for digestion. 



Not all the Trypanosomes, however, arc able to leave tlie gnat. Tliose 

 which become attached to the epithelium of the stomach, instead of to that 

 of the oesophageal invagination, are not carried backwards to the colon, and 

 60 into the circulation, but remain behind in the mid-gut. These arc 

 chiefly females, and they can produce a general recurrence of the parasites 



' A ring of muscular tissue, by means of which the cavity of the pharynx 

 can be shut off from that of the oesophagus. 



