62 THE ANIMAL PARASITES OF MAN 



up into eight small trypanosomes. These microgametes are dis- 

 tinguished from the other forms not only by their diminutive size, 

 but by the development of their locomotor apparatus. They have 

 large blepharoplasts and very long flagella, which render them 

 exceedingly motile. 



When the blood has been entirely digested and the midgut 

 remains empty, the parasites attach themselves to the epithelial 

 cells lining the posterior part of the stomach about the valve in 

 front of the malpighian tubes. Here they agglomerate in large 

 clusters. When very numerous the parasites may greatly injure 

 the walls of the midgut and thus cause the death of the mosquito. 

 If the infection is followed by a long fast, all the asexual and 

 male forms die oif, the female forms alone persisting. When the 

 mosquito again sucks blood the parasites detach themselves, dis- 

 tribute themselves in the blood, and then pass into the narrow 

 anterior portion of the midgut, where they form a large cluster. 

 At the time of the next feed the whole cluster is pushed back by 

 the inrushing blood right through the midgut into the ileum, 

 thence, by the peristaltic contractions of the intestine, the cluster 

 is passed into the colon as far as Basili's curvature, the narrowest 

 section of the intestine. On account of the nature of the epithe- 

 lium in this part, the parasites are able to penetrate through the 

 intestinal wall. Thus they reach the circulation, and through the 

 dorsal vessel some are carried forwards into the lacunoma round 

 the pumping organ, others are carried to the ovaries. Those 

 carried forwards come to rest between the pumping organ 

 and the pharyngeal valve ; here they form a thick cluster, and 

 by injuring the tunica elastico-muscularis, which is very delicate 

 at this part, they pass into the pharynx. They are thence passed 

 into the blood of the alternative host during the evacuation of 

 the sucking reservoirs. The entire period of the migration of the 

 parasites from their entry into the body of the mosquito to their 

 exit from the latter, occupies a period of from seven to eight days. 



The parasites that attain the ovaries penetrate the youngest 

 eggs, and the macrogametes go to rest within the vitellus. The 

 latter remain in the gregarine condition during the entire develop- 

 ment of the embryo. They subsequently pass into the larva, and 

 are found also in the perfect insect. The mosquito recently 

 emerged from its pupa case does not seem able to transmit the 

 infection by means of its first bite. According to Schaudinn's 

 observations it can infect birds only at the time of its third feed. 



