11 DEVELOPMENT OF OVA Ta 
EKutherian mammals, including the Marsupials, are very small as 
compared with those of any other vertebrates, excepting only 
Amphioxus, where the young are hatched early as free swimming 
larvae. They also differ in a highly characteristic way in the 
mode of their development within the ovary. These processes 
are to some extent illustrated in Fig. 44. The main frame- 
work of the ovary is formed of the so-called “stroma,” which is 
a mass of tissue formed of more or less connective-tissue-like 
cells. Within this are numerous cavities, the Graafian follicles. 
The very young follicles consist of but a single layer of follicular 
cells surrounding the ovum, which lies centrally. The follicular 
SOAOOGEE:: 
Ty NG 
Gayo te fee 
eft Z 
aeage = 
Fic. 45.—Two stages in the development of the Graafian follicle. A, With the folli- 
cular fluid beginning to appear; B, after the space has largely increased. caps, 
Capsule ; disc, cumulus proligerus ; memb, membrana granulosa; ov, ovum; sp, 
space containing fluid. (After Hertwig.) 
cells gradually increase in number until the ovum lies in the 
midst of several layers of cells. At this period a vacuity is 
formed between some of these cells, and grows into a large 
cell-free cavity ; the ovum does not lie loosely in this space, but 
is connected at one side with the follicular cells, which still line 
the interior of the Graafian follicle by the so-called discus 
or cumulus proligerus. The ege or ovum has, moreover, a layer 
of cells immediately surrounding itself. All these facts can be 
gathered by an inspection of Fig. 45. It has been shown 
that, as in lower vertebrates, the cells immediately surrounding 
the ovum afe connected with it directly by delicate processes 
which penetrate the actual membrane of the egg. 
The only ova which depart at all in structure from that above 
described are those of the Monotremata. The credit of this 
