CHANGES IN THE OVARY 125 



one ovum whose condition happens to be the most vigorous. 

 " This cannibalism on the part of the young ovum is not sur- 

 prising, if the life of an ovum be considered. It is really but 

 the normal condition of the cell at all its stages of development ; 

 it grows and fattens at the expense of other cells. In the 

 young ovary, it is starting its first stage of growth and must 

 devour other cells ; later on, during the growth of the follicle, 

 it lives upon the follicle-cells, and later still, when, after fertilisa- 

 tion, the [term] ovum in its extended sense refers to the young 

 foetus, [this latter] lives on the material provided by the cells 

 of the maternal organism." l 



MATURATION AND OVULATION 



The youngest and smallest Graafian follicles lie near the 

 surface of the ovary, but pass inwards as they increase in size. 

 The large, mature follicles, however, come to lie just below the 

 surface from which they begin to protrude visibly at the ap- 

 proach of the breeding season. During the prooestrum one or 

 more follicles (the number varying in different animals, accord- 

 ing to the size of the litter) may generally be seen showing a 

 very considerable protrusion, while in some animals, such as the 

 sow, the appearance of the ovary at this time is not dissimilar 

 to a bunch of grapes. 



A large Graafian follicle in a mature ovary contains the 

 following parts : Forming the outermost part of the wall and 

 in continuity with the ovarian stroma is the theca externa, 

 which is a layer of somewhat fibrous connective tissue. Within 

 this is the theca interna, which is less fibrous. The two thecse 

 are only slightly modified ovarian stroma. Within the theca 

 interna is the epithelial wall, which, in the very young follicles, 

 consists of a single layer of cells immediately surrounding the 

 ovum. These, as already mentioned, multiply rapidly (by 

 mitotic division) and give rise to a layer many cells deep, which, 

 as the follicle increases in size, becomes divided into two layers, 



1 Lane-Claypon, " On Ovogenesis," &c., loc. cit. That one ovum may de- 

 velop at the expense of others is particularly well shown in Hydra, Tubularia, 

 and certain other Coslenterates. The nuclei of the ingested ova continue 

 to be easily recognisable even during the early segmentation stages of the 

 developing egg. 



