Abstract
The ovaries of a minke whale (Balaenoptera acutorostrata) and Risso's dolphin (Grampus griseus) were studied histologically to determine the maturity and activity of the oocytes and associated structures. The minke whale, 562 cm in the total length (TL) with ovaries of 11.2 and 12.0 cm in length, was washed up on the coast of Ishiji, Nishiyama-machi, Niigata Prefecture, and the Risso's dolphin, 269 cm TL with ovaries of 4.5 and 5.0 cm, near the mouth of Miomote River, Murakami City, Niigata Prefecture. With the exception of one lobe in Risso's dolphin, which had an unusual projection comprising connective tissue only, the ovaries of both specimens had smooth dorsal and ventral surfaces. Histological examination revealed that most of the oocytes had reached the primary or secondary follicle stages. However, Graafian follicles were seldom detected, and corpora lutea and albicantia tissues not at all. Numerous atretic follicles in various degenerative stages were apparent. The overall picture, together with macroscopic observations, was of young immature animals.
https://ror.org/04ww21r56 
