398 
Tetrarhynclius erinaceus 
other. Other cells in the interior of the follicle are entirely undiffer¬ 
entiated, are small and stain faintly: they are evidently cells which are 
taking no active part in actual spermatogenesis. 
The distinctive character of the follicle at this stage is the presence 
of relatively large capsular structures about 0’02 mm. in diameter ( 8 ) 
consisting of an outer single layer of cells similar in character to those 
lettered 7 . These cells are always arranged loosely and are hardly in 
actual contact with each other, so that it is sometimes difficult to be 
sure that they form an actual capsule and are not simply the limits of 
a space formed among the indifferent cells of the follicle. Within this 
capsule is an ovoid body which is about 0 016 to 0‘018 mm. in diameter, 
and which is separated from the capsule by a space. It is coarsely 
granular in structure and has small projections on its periphery. 
The above structures are all that are present in the maturing 
follicle. 
All the follicles in the testis represented by PI. XXI, fig. 10 are ripe 
and contain fully formed spermatozoa. They are now larger and looser 
in texture than in preceding stages. PI. XXII, fig. 13 represents such a 
ripe testis cut longitudinally and it will be seen that considerable changes 
have taken place. The outer limiting membrane still exists but the 
layer of marginal cells has completely disappeared. A number of the 
small indifferent cells ( 7 ) are scattered throughout the follicle and are 
embedded in a granular debris produced by the breakdown of some of the 
tissue elements. The large ovoid capsular structures still exist but they 
are undergoing degeneration, for the cells composing their walls are now 
further apart as if the structure were breaking up. The morula-like 
structure in the centre is much smaller, is homogeneous in nature and 
the space between it and the outer irregular layer of cells is filled with 
granular debris. The characteristic feature of this phase is the presence 
of large bundles of fully formed spermatozoa. 
Spermatogenesis. It would be unprofitable in the meantime to 
discuss these appearances in the light of already published accounts of 
spermatogenesis in other Cestodes—before doing so a greater number 
of proglottides would have to be examined after fixing and staining 
by a variety of methods. But it is probable that the course of the 
formation of spermatozoa is much as follows:—the undifferentiated 
cells which are arranged round the periphery of the follicles in the 
youngest stages proliferate into the interior and give rise to two 
kinds of cells, the small indifferent cells, ( 7 ), and the large nuclei (/3). 
The latter divide actively forming the morula-like structures, which 
