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guishable. In species containing relatively small eggs the thickness of 
the follicle increases with the growth of the egg. Examples of this are 
found in fishes, but the extreme is found in the Graafian follicle of 
mammals. 
The Graafian follicle, whose structure is well known, offers some pecu- 
liar features which may be explained in connection with the reduction of 
the size of the ovum in the mammalian phyllum. Suggestions as to how 
any why the many layered follicle of higher mammalia has arisen from 
the single layered follicles of monotremata are furnished by the viviparous 
fish Cymatogaster. The egg in this species is much below the average in 
size. It has in fact lost nearly all of its yolk. It is significant, therefore, 
that among many individuals with normal follicles there are occasionally 
found individuals containing a small number of many layered follicles. It 
is to call attention to these and to compare them with the mammalian 
structures that the present article is prepared. 
In order to fully appreciate the conditions, follicles of various mam- 
mals and yarious stages in the same mammal may be briefly mentioned. 
Poulton has found that in monotremata the follicular epithelium remains 
a single row of cells. 
The follicles of the cat, after moving from the surface toward the 
deeper parts of the ovary, begin to thicken from the single layered stage 
to the many layered stage. Fig. 1 represents the single layered stage. 
Fig. 2 represents the many layered stage in which the follicular cavity is 
beginning to be formed. The egg in the many layered stage is usually 
imbedded in the portion of the granulosa nearest the surface of the ovary. 
The first traces of the follicular cavity are seen in the thicker part of the 
granulosa. As the egg grows the follicle cells still multiply and the liquor 
folliculi filling the cavity distends the latter. Fig. 3 shows the follicle 
with its now nearly ripe egg imbedded in the discus proligerus. This 
figure represents the typical Graafian follicle. 
The follicle of the rabbit differs from that of the cat (Fig. 4). Columns 
of granulosa cells connect the outer mass of cells with that surrounding 
the egg. In such follicles as many as four stalks are found in a single 
section. On examining other sections of the same follicle other stalks 
may be found so that a single follicle may contain as many as seven of 
these. In both the cat and rabbit the thickness of the follicular wall is 
not reduced with the growth of the egg, but rather increases proportion- 
ally with the growth. In the opossum both types of follicles of Figs. 
