THE REPRODUCTIVE ORGANS 387 



adult at about 20-0 m.) also had large follicles. This predominance of ripening follicles 

 during the southern winter argues in itself a period of sexual activity, and the increase 

 in size of the follicles of the three whales exceeding 19-0 m. suggests the approach of 

 sexual maturity. 



-1 r — r 



MAR. APR. MAY JUNE JULY AUG. SEPT. OCT. 



Fig. 124. Fin whales. Size of the largest ovarian follicles during the year. 

 • Mature females neither pregnant nor lactating. o Immature females longer than 19-0 m. 



(c) The Corpus Ltiteum of Pregnancy 



It is from the condition of the corpora lutea that the most important conclusions can 

 be drawn as to the sexual condition and history of the whale. It will be convenient 

 to start with an account of the corpus luteum of pregnancy. 



When an ovum is shed the follicle from which it was liberated becomes a corpus 

 luteum by inward growth and hypertrophy of the follicular epithelium, carrying with 

 it blood-vessels from the surrounding tissue (see Marshall, 1922, who discusses in 

 detail the physiology of the ovaries and gives references to original work on the subject). 

 If fertilization occurs, the corpus luteum persists to all intents and purposes in its 

 original condition throughout the period of gestation, but if pregnancy does not super- 

 vene it persists for a comparatively short time and then begins to undergo involution. 

 In whales of all the species examined the corpus luteum of pregnancy is a very large and 

 conspicuous body (Plate XL, figs. 3 and 4) with a scar marking the point of rupture 

 of the follicle. The scar, which is sometimes of considerable size, consists of a dimple 

 about 5 mm. in diameter surrounded by a raised area which may be called the " corona " 

 and which may measure as much as 6-o cm. in diameter. Internally the corpus luteum 



17-2 



