Examination of the ovaries from 106 different species of mammals has revealed the presence in a number of them of sub-surface crypts, lined by germinal epithelium, which penetrate into, or pass through the tunica albuginea. Such crypts are differentiated from intra-ovarian clefts which simply sub-divide the ovary and from Pfluger's tubes which have no lumen. The crypts are constantly found in six species of Pinnipeds, frequently in numbers of Marsupialia, Insectivora, many fissiped Carnivora, two species of Procaviidae, Loxòdonta africana, and occasionally in Rodentia and Primates, including the human subject. In the Pinnipedia, and probably in the other species, the maximum development is at the time of oestrus and early pregnancy and the degree of development of the crypts appears to undergo changes corresponding to the degree of follicular activity and oogenesis. Experimental injection of gonadotrophins or oestrogens causes the appearance of crypts in Putorius furo, a species which does not normally show marked crypt formation.

The association of crypt formation with oogenesis is discussed and it is suggested that such crypts are possibly a vestige of the primitive pattern of ovarian structure.

In the human subject the formation of crypts is associated with the possible subsequent development of fibroadenoma or cystadenoma of the ovary.