DEVELOPMENT OF OVA 



71 



Eutherian mammals, including the Marsupials, are very small as 

 compared with tliose of any other vertehrates, excepting only 

 Amjihioxus, 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 t)var}\ 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 conuective-tissue-like 

 cells. Within this are numerous cavities, the Graafian follicles. 

 The very young follicles consist of liut a single layer of follicular 

 cells surrounding the ovum, which lies centrally. The follicular 



ca/>s .<=i=s^^s 



disc 



Fig. 4.5. — Two .stages in the development of the Graafian follicle. A, With tlie folli- 

 cular fluid beginning to appear ; B. after the .space has largely increased, caps, 

 Capsule ; disc, cuiunlus proligerus ; vwiiih. inenibrana granulosa ; ov, ovum ; sji, 

 space containing fluid. (After Hertwig.) 



cells gradually increase in numlx'r until the ovum lies in the 

 midst of several layers of cells. At this period a vacuity is 

 formed jjetween 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 egg 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 are connected with it directly by delicate processes 

 which ]jenetrate the actual membrane of the egg. 



The only ova which depart at all in structure from that aljove 

 described are tliose of the Monotremata. The credit of this 



