172 BULLETIN OF THE 
In some specimens cross sections through the middle or anterior 
region of the body show that the sac is moderately filled with cells 
whose nuclei stand close together and are in a kinetic condition. Here 
the ventral boundary of the dorsal line seems to be wanting in places, 
as if the cells in the sac were directly connected with those of the dorsal 
line, while the wall of the sac is laterally directly continuous with the 
basement membrane, which covers the line ventrally. One can some- 
times find among the cells in this region stages which seem to show a 
transition between the kinetic nuclei and the groups of spermatozoa 
found among them ; but this condition was encountered in only a single 
specimen, and the material was not in sufficiently perfect histological 
condition to allow a study of spermatogenesis. This anterior portion of 
the sac I regard therefore as testis, and the posterior crescent-shaped 
portion as at once receptaculum and vas deferens. In the stage in which 
the kinetic nuclei were found in the anterior portion of the sac, the walls 
of the posterior portion were collapsed, and hardly a single spermatic 
element was to be found in it. This is the youngest stage which I have 
studied. 
In another, older stage the sac was filled from end to end with the 
deeply stained highly refractive spermatozoa, and so enlarged that it oc- 
cupied nearly the entire body cavity. Finally, in the oldest stage found 
the sac (Fig. 11) appears in the anterior part of the body as a mere 
remnant with collapsed walls containing an occasional spermatozodn. 
At the tail, however, a small quantity of spermatozoa was collected near 
the terminal orifice. The diminished thickness of the protoplasmic zone 
in the body wall shows this individual to have been comparatively old. 
In one specimen in which the sac was thus collapsed, however, the body 
wall was moderately thick. In these cases copulation seems to have 
taken place, and the few spermatozoa are merely remnants of the original 
contents of the sac. 
The organ described by Biirger on pages 646, 647, is evidently the 
same as this, and his belief that it was a testis rather than an ovary is 
confirmed by the preceding account. The description he gives of the 
organ either shows that the specimen studied by him was intermediate 
between the first and second stages here described, or else was based 
upon different individuals and represents different stages. 
In one of my individuals which, to judge from the thickness of the 
body wall, must have been young, there were clusters of polyhedral cells 
here and there in the anterior portion of the body cavity, and these 
clusters were crowded full of spermatozoa in small bunches, as if they 
