OVARIAN FOLLICLES IN FERRETS AND FERRET-POLECAT HYBRIDS. 311 
called the membrana limitans interna , is formed on the inner surface of the follicular 
epithelium and on the projecting part of the cumulus. It consists, mainly, of a con- 
densation of the stroma of the primary liquor folliculi, and in it are incorporated 
the remnants of degenerated follicle cells (fig. 17, PL III ; figs. 52, 55, 57, PI. IX). 
In the earlier stages of its formation the membrana interna is continuous, towards 
the interior of the cavity, with the stroma of the primary liquor folliculi, and, 
externally, it is connected with protoplasmic processes of the more internal follicle 
and cumulus cells. 
As the membrana limitans interna forms, the stroma of the more central part of 
the primary liquor folliculi is broken up into a granular detritus, and, under the 
influence of some of the fixative fluids, especially Zenker’s fluid, the remnants of the 
stroma and the fluid in which it lies tend to form a granular coagulum. In some 
cases the more solid part of the coagulum separates from the more fluid part without 
any disturbance of the connection of the membrana interna with the follicle and 
cumulus cells, but in other cases, as the coagulum shrinks the membrana interna is 
torn away from its peripheral attachments, and always more from the follicle than 
from the cumulus cells. It is probable that the shrinkage of the coagulum and the 
attendant displacement of the membrana interna are not normal processes but are 
due entirely to the reagents used, but I can offer no direct proof in support of 
this supposition. 
At an early stage in the growth of the follicle an external limiting membrane, 
the “ basal membrane ” of many investigators, appears at the outer periphery of the 
follicular epithelium ; it is quite distinct when the follicular epithelium consists of 
a single layer of more or less well-defined cubical cells, and it persists until shortly 
before the rupture of the follicle, when it is broken up by the penetration of blood- 
vessels into the follicular epithelium (figs. 49, 50, PL VIII). 
As the follicle reaches the full growth which it is capable of attaining, inde- 
pendently of insemination, a growth which may, for convenience, be termed the 
pre-inseminal growth, the outlines of the follicle cells, which were more or less 
indefinite in the early stages, become more distinctly defined. At this stage the 
outermost cells of the general epithelial lining and the cells adjacent to the oolemma 
in the cumulus are columnar, conical, or spindle shaped (figs. 52, 54, 55, 56, 57, Pl. IX ; 
figs. 58, 60, Pl. X; fig. 53, Pl. IX; fig. 16, Pl. III). The outer ends of the follicular 
epithelium cells, which are always the more distinctly defined, are attached to the 
external limiting, membrane, and the inner ends blend with processes of the more 
internal cells, which assume stellate and irregular outlines, to form a cellular reticulum. 
It is to the inner portion of this cellular reticulum that the internal limiting 
membrane is attached. 
The only further change of importance which takes place, in the follicular 
epithelium, during the pre-inseminal growth of the follicle, and which occurs as the 
growth is completed, is the separation of the cumulus cells into an ovular layer next 
TRANS. ROY. SOC. EDIN., VOL. LII, PART II (NO. 13). 50 
