408 
Tetrarhynchus erinaceus 
cells and the testicular follicles. In the youngest proglottides the 
vitellaria are diffuse—thus they are represented in PI. XXIII, fig. 21, as 
groups of cells lying quite loosely in the parenchyma just internal to the 
subcuticular cells. In later stages, however, they become compact and in 
PI. XXII, fig. 12, which represents a part of a section of a mature segment, 
they are seen to be composed of distinct follicles. The vitelline glands 
originate in the very young segments as cords of cells, which later on 
acquire a very thin limiting membrane from the secretion of the adjacent 
parenchyma; and then become arranged to form short tubular follicles 
connected by duct-like structures. The terminal follicles are arranged 
radially so that they run out from the axis of the proglottis in all 
directions towards the periphery. The conducting portions of the 
follicles run towards the axis of the segment and join together to 
form longitudinal collecting ducts. But there is never any absolute 
distinction between the secreting and conducting parts of the gland 
except that the latter are generally a little less in diameter than 
the follicular parts. All parts are, however, filled with the yolk 
cells. 
PI. XXII, figs. 12 and 17, represent small parts of the vitelline gland 
under a higher magnification and it will be seen that the cells contained 
in the follicles in the earliest stages are surprisingly like the cells in the 
ovary, so much so that it is sometimes difficult to distinguish between 
the two kinds, especially as the limiting membranes of the two organs 
are so tenuous that it is occasionally difficult to trace them. Fig. 17 
represents one of the vitelline follicles cut in longitudinal section. 
There is air outer limiting membrane and inside this there are usually 
two kinds of cells. One kind are small and resemble the ovarian cells 
except that the chromatic synapse is not so prominent. The other 
kind includes cells which are much larger, and are polyhedral in shape 
because of their mutual pressure against each other. They have large 
nuclei, faintly staining nucleoli, and the cell substance is almost entirely 
composed of coarse brightly staining granules—obviously yolk substance. 
In a section of one of these follicles there are usually about ten to twenty 
of these cells in a field. The small cells are similar to those in the 
peripheral parts of the very young proglottis: they are obviously still 
almost entirely undifferentiated. 
Posterior to the ovary the collecting ducts of the vitelline glands 
run transversely to the long axis of the proglottis and then join to¬ 
gether near the middle of the segment. These ducts are greatly enlarged 
and may be spoken of as a yolk reservoir. From this enlarged part a 
