463 



rectum. The narrow end of each is attached by a thin 

 stalk, which is hollow, to the ventral part of the ninth 

 abdominal segment, and the whole organ lies vertically to 

 the longitudinal axis of the larva (fig. 185). 



The testis is covered by a membrane consisting of rather 

 flattened cells — ^the ''serosa," or reflected abdominal "peri- 

 toneum." Lying within the sac so formed is a great mass 

 of very closely packed spermatogonia, somewhat rounded cells, 

 measuring about 6/x in diameter. Each contains a large clear 

 nucleus, the chromatin of which is concentrated into a small 

 heavily staining karyosome ("vesicular" type of nucleus) 

 (fig. 186). Cell division does not appear to be going on at 

 this time. 



Supporting these spermatogonia is a fine connective net- 

 work, very difficult to detect; it consists essentially of a 

 number of branching cells, not unlike vertebrate nerve cells 

 in appearance (fig. 186), and somewhat smaller than the 

 spermatogonia, the network being formed by the junction 

 of adjacent cell processes. 



During larval life the spermatogonia increase in number, 

 the testes in the defaecating larva being in the form of two 

 rounded organs, much longer than the testis of the first larval 

 instar. The spermatogonia have not increased in size; indeed, 

 they are somewhat smaller than those occurring in the first 

 instar, being now about 4j/x in diameter. The connective 

 tissue network has become more prominent. 



The "stalk" of the organ, which is now definitely recog- 

 nizable as a vas deferens, has increased considerably in length ; 

 its wall consists of a single layer of cubical cells, covering 

 which, of course, is the serosa. The lowest portion of the 

 vas deferens now begins to dilate. The cells lengthen greatly, 

 and change from cubical into elongated columnar cells. It 

 is the rudiment of the vesicula seminalis, and is already well 

 developed in the larva twelve hours after defaecation. It 

 lies in close contact with the proliferating cells of the in- 

 vaginated ventral part of the ninth abdominal segment, from 

 which, as above described, the penis is beginning to develop. 

 But its cavity does not yet possess any communication with 

 the exterior. 



At this stage also (twelve hours after defaecation) the 

 action of the connective tissue in the testis is clearly visible, 

 resulting in the binding together of the spermatogonia in 

 little groups of twenty to thirty, all clustered tightly 

 together. By the time the larva pupates, these clusters of 

 sperm otogonia have loosened considerably; the connective 

 tissue cells and network are clearly visible. Sometimes the 

 connective tissue undergoes considerable hypertrophy at this 



