118 Embryonic Development of Ovary and Testis of Mammals 



der. The ground substance is a rather compact connective tissue in 

 which are imbedded the slender transversely-placed medullary and 

 rete tubules, which are quite inconspicuous and devoid of a lumen. 

 Large open lymph spaces are formed in the stroma. Typical interstitial 

 cells are not at all uncommon. 



75 Days after Birth. Ovary. — Certain of the innermost follicles 

 have increased greatly in size, having in some cases almost reached 

 maturity. They encroach upon the limits of the medullary region to 

 such an extent as to make the latter band-like in cross-section. In 

 Plate VII, Fig. 26, is represented a portion of the ovary showing the 

 more important tissues composing it. The granulosa cells have well- 

 defined boundaries, being polygonal in shape as the result of mutual 

 pressure. The nuclei are rounded and are found to divide by mitosis. 

 The follicle is bounded externally by a clearly-defined membrana pro- 

 pria, external to which one finds a very thin layer of attenuated con- 

 nective tissue cells. This investment (follicular capsule) is similar 

 to that which surrounds the seminiferous tubules. 



Outside of this occurs the theca interna, which is from one to four 

 cells thick, the component cells being elongated in a direction parallel 

 to the surface of the follicle. They are rich in cytoplasm. The large, 

 rounded or oblong nuclei stain more lightly than do the nuclei of the 

 granulosa ceils. 



The slender branching and anastomosing medullary and rete cords 

 are distinguishable from the surrounding stroma by the clear cyto- 

 plasm and small oblong, deeply-staining nuclei of their cells which 

 have apparently remained unchanged in character from the earliest 

 stages onward. 



Certain young follicles of a stage just before the formation of the 

 follicular cavity have begun to degenerate, the process first afllecting 

 the oocyte which in some cases has disappeared wholly or in part. The 

 mass of follicular cells becomes irregular in outline, but shows no 

 signs of degeneration. It is quite likely that certain cords of cells 

 with nuclei larger than the true nuclei of the medullary cords in the 

 midst of which they lie, have originated from these degenerating folli- 

 cles, the resemblance between their nuclei and those of the normal 

 follicles being very striking. 



85 Days after Birth. Ovary. — The ovary as a whole has changed 

 but little in form and size, this stage being most remarkable on ac- 

 count of the great increase in the number of interstitial cells. This 

 increase is due to a very extensive process of follicle degeneration 

 which seems to be at its height, affecting follicles in all stages of de- 



