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PROFESSOR ARTHUR ROBINSON ON 
the oolemma, and a more external layer next the cavity of the follicle and the base 
of the cumulus, by the appearance of fluid-filled spaces between the two groups of 
the cumulus cells (fig. 16, PL III; fig. 53, PI. IX). The appearance produced in 
the cumulus by these changes is not dissimilar to that seen during the formation of 
the primary liquor folliculi, but there are differences of importance. During the 
formation of the primary liquor folliculi the spaces which appear amidst the folli- 
cular cells are separated from one another, from the first, by thin strands of 
protoplasmic substance, which become finer and finer as the cavity of the follicle 
enlarges. The spaces which appear in the midst of the cumulus cells, and which 
contain the fluid 'part of a new fluid, the secondary liquor folliculi, have at first a 
very similar appearance to those associated with the formation of the primary liquor 
folliculi, and they are similarly separated by fine strands of protoplasmic substance, 
but the protoplasmic strands are more granular and less' filamentous than those of 
the primary liquor folliculi ; moreover, as the cells of the basal part of the cumulus 
are pushed apart by the increasing fluid, masses of granular substance are formed 
which are not seen during the formation of the primary liquor. These masses are 
devoid of nuclei, they are surrounded by groups of fluid-filled spaces, and they are 
connected with the protoplasm of the adjacent cells by granular protoplasmic 
strands (fig. 40, PI. VII; fig. 48, PI. VIII). The mode of formation of the proto- 
plasmic masses is uncertain ; they must be evolved from the protoplasm of the 
cumulus cells, but there is no evidence of the breaking down of cells, and no evidence 
of the migration of nuclei from the masses to adjacent parts. Eventually, after 
insemination, the protoplasmic masses break down and disappear. Another difference 
between the primary and the secondary liquor folliculi is, that the secondary liquor 
does not coagulate and shrink under the influence of certain fixatives as the primary 
liquor does in the later stages of the growth of the follicle. 
The Sheaths of the Follicles. 
It is customary to state that each fully developed ovarian follicle is surrounded 
by two sheaths, the theca interna and the theca externa. The theca interna, which 
lies immediately outside the external limiting membrane, is said to consist of a 
somewhat loose cellular stroma interspersed with blood-vessels and with plasma 
cells, which, apparently, is the name applied by some authors to what are un- 
doubtedly interstitial cells. The theca externa is described as being denser, more 
fibrillated, and less vascular than the theca interna, and as containing few or no 
interstitial cells. 
In the ferret the stroma surrounding follicles, which have attained their full 
pre-inseminal growth, is sometimes arranged in the so-called typical manner ; more 
commonly, however, the circumfollicular stroma is not clearly separated into an 
inner and an outer theca. It is a fibro-cellular layer of very varying thickness, 
