§ I STRUCTURE OF OVARY 79 



the continuous formation of short-lived hormonal glands, 

 and (3) reparative, through the restitution of the different 

 structures according to the losses sustained. 



Microscopic investigation of a mature follicle (Fig. 62) 

 reveals the following structures: (i ) The ovum is surround- 

 ed by a membrane, the oolemma (zona pellucida). Upon this 

 lies an inner follicular layer, (2) the granulosa, consisting of 

 a single layer of cells. The granulosa cells hang together by 

 means of very broad plasmodesmas. Outside the granulosa 

 there is a wide-meshed layer of cells, (3) the theca interna. 

 The cells composing the latter hang together by means of 

 fine plasmodesmas. Between the cells there are spiral-shaped 

 collagenous fibrils, lymphatic cavities and capillaries. Finally, 

 the follicle is enveloped by (4) the theca externa, consisting 

 of spool-shaped cells. 



The ova develop in two different places in the ovary, i.e., 

 caudally and cranially. These arc the oogenetic fields, where 

 a rapid production of ova takes place at certain times. In the 

 outer zones of these oogenetic fields the following processes 

 may be observed (Fig. 63). The nuclear epithelium, con- 

 sisting of a single layer, has quite a uniform appearance out- 

 side the oogenetic zone. Nearer the boundary of the field the 

 epithelium is more active; it becomes raised, and the nuclei 

 swell into small cysts. Thereupon follows the first visible 

 differentiation: some of the cells enlarge, their nuclei become 

 spherical in shape, possess a large, nucleolous and linin net- 

 work with numerous chromatin islands; the cytoplasm also 

 dilates, numerous vacuoles and granula appear. These enlarged 

 cells later become the ova. Simultaneously, the neighbouring 

 cells undergo a different change. Their nuclei become extended, 

 and acquire a fine network of linin threads and only little 

 chromatin. Through the pressure of the neighbouring grow- 

 ing cells both the nucleus and the cytoplasm become flattened, 

 and the cells place themselves like bonnets against the growing 

 cells. These cells also seem to have a certain mobility, for they 

 sometimes move to the base of the ovum. These attendant 

 cells become the future granulosa cells of the follicle. On close 

 examination of the boundary zone an alternating develop- 



