J. Johnstone 
397 
We may now consider these various organs in greater detail. 
The testes. I have called the proglottis represented in PI. XXI, 
fig. 10 an immature one in relation to the development of the fertilised 
ova, but it really represents the maximum degree of development of 
the male organs. As the segment grows in size the male apparatus 
degenerates and only then do the female oi’gans attain their greatest 
development. In the phase under consideration there are about 
300-400 testes in a single segment, and they occupy a considerable 
proportion of the bulk of the latter. It is difficult to count their exact 
number or to represent them all accurately in one drawing as they are 
partially superposed on each other. Each is a short cylinder tapering 
towards one end (Text-fig. S) and placed with its long axis in a 
dorso-ventral direction. They lie in the central part of the segment but 
there are always a number at the anterior end in front of the uterus. 
They are completely surrounded by the vitelline glands and dorsal and 
ventral to them are the longitudinal muscles of the proglottis. Each 
follicle is about 0'2 mm. in length, and about 0'05 mm. in diameter. 
The testicular follicles in the youngest segments in which they can 
be identified are about 0'02 mm. in diameter. Each consists of a mass 
of undifferentiated cells, and there are usually about ten to twenty of 
these in a transverse section. There is no testicular membrane in these 
youngest testes, but as development proceeds a very delicate limiting 
membrane makes its appearance. The cells in the interior now become 
more numerous, the follicle grows, and differentiation begins. PI. XXII, 
fig. 18 represents an immature testicular follicle and we see that there 
is an outer epithelial layer consisting of rounded cells which are not in 
close contact with each other, while within this are two kinds of cells. 
One kind, 7 , resembles those of the periphery and are still undifferentiated; 
but the others, /3, are evidently actively dividing. I have been unable 
to see any actual division figures but the material at my disposal was not 
such as would show these easily. These dividing nuclei are larger than 
the other cells and have very prominent chromatic synapses. 
PI. XXII, fig. 15 represents an older follicle from a riper proglottis 
and is cut longitudinally. There is now a very evident outer limiting 
membrane and the peripheral layer of cells has broken up, though 
undifferentiated cells of the sanre kind are to be seen in groups round 
the margin : these cells are about 4 yu. in diameter and they always lie 
in groups against the outer membrane. At /S can be seen cells similar 
to those so lettered in fig. 18 and like the latter they are large and have 
prominent chromatic reticula. They usually lie separately from each 
