THE STEUCTURE OF THE TESTES. 699 



The more the ramification progresses, the thinner and more deli- 

 cate do the twigs become, until they finally escape detection. Only 

 in isolated cases can they be seen uniting -with the testes (which then 

 appear stalked), and especially with those which lie nearest the median 

 area. They have, as a rule, a comparatively straight course, and 

 frequently seem, especially near the seminal cistern, to result from a 

 dichotomous branching. 



The Testes themselves exhibit exactly the same structure and dis- 

 position as in the large-jointed Tmnim. The vesicles have a similar 

 irregularly oval or spherical form, and have when full-grown an 

 average diameter of 0'1-013 mm. They extend in a thick layer over 

 the lateral portions of the middle layer (Fig. 361), and are here and 

 there insinuated between the loops of the uterus. Towards the lateral 

 borders they are more closely grouped, and the transverse diameter 

 becomes diminished in consequence. Stieda has estimated their 

 number at about 320-400 in each joint, while Landois and Sommer 

 compute it at 1000-1200. According to my estimate, the former 

 is below, the latter above the mark. In each lateral area I count 

 twenty-three in a transverse line, and sixteen in a longitudinal, and 

 thus compute a total of between 600 and 700, which agrees well with 

 Eschricht's estimate of 700. 



Both Landois and Sommer and Moniez describe the testes as simple 

 lacunae in the tissue of the middle layer, but with reagents I can dis- 

 tinguish the coagulated contents from a distinct, delicate, enveloping, 

 cuticular membrane. The striated substance within the mature 

 vesicles consists of extremely fine long spermatozoa, which are grouped 

 in numerous bundles, or rolled up, and which (according to Stieda 

 and Sommer and Landois) bear at one end a strongly refringent head. 

 Between these can be seen separate aggregates of small (0'005 mm.) 

 nuclear cells (Kernzellen^), which occur in much greater abundance in 

 the unripe vesicles, where they may, indeed, form the whole contents. 

 It is hardly necessary to remark that these cells in course of time 

 develop into spermatozoa. Their occurrence in the ripe testes also 

 shows that the formation of spermatic elements in Bothriocephalus 

 is not, any more than that of the ova, restricted to a definite period, 

 as in the Tcenice, but continues as long as the vegetative life of 

 the joints. ^ 



The dark balls which not unfrequently occur in the ripe joints 

 between the coils of the uterus and of the vas deferens, are, in spite of 



^ Deceived by these cells, Botticher {loc. cit., p. 121) thought that each sperm-filled 

 testis was a coil of fine canals, with an internal cellular lining. 



" According to Moniez, the spermatozoa are formed freely in the tissue of the tape- 

 worm, roll themselves up in spherical masses, and maie their way independently to the 

 ramifications of the vas deferens .(Zoc. citt, p.. 147). „_ 



