360 WHALES 



mammals the corpvis luteum is frequently larger than the rest of the ovary. 

 The most superficial observation will show that the surface of the corpus 

 luteum displays an annular structure of diameter up to 2 J inches, sur- 

 rounding a hollow of diameter about ^ inch. The hollo^v is the spot where 

 the follicle has ruptured to release the ovum. In other words, the corpus 

 luteum is simply a ruptured follicle whose wall has greatly increased in 

 size and formed new tissue. It is this tissue which produces progestin, a 

 hormone which stimulates and strengthens adhesion of the fertilized ovum 

 to the uterine wall. The corpus luteum of big whales yields from thirty to 

 forty milligrams of progestin per kilogramme of organ, and whale pro- 

 gestin, together with whale oestrin (a hormone produced by the cells 

 lining the maturing follicle) , is in fact put to good use by the pharmaceutical 

 industry in a number of countries. 



After ovulation, the follicles of all mammals produce corpora lutea 

 which, as we have seen, swell to an immense size if the ovum is sub- 

 sequently fertilized. In dogs, cats, pigs and other mammals, the corpus 

 remains large and active right up to the end of gestation, but in horses, 

 cattle and sheep its function is taken over by the placental tissue about 

 halfway through pregnancy, when the corpus degenerates. Degeneration 

 also occurs whenever ovulation is not followed by fertilization and preg- 

 nancy, but in that case the corpus begins to undergo recessive changes 

 some ten days after ovulation. In either case, the glandular yellow (or 

 pink) tissue quickly disappears until no more than a fairly degenerate type 

 of white connective tissue (hyaline-sclerotic tissue) remains, the so-called 

 corpus albicans (white body) (Figs. 199 and 200). In big whales the 

 corpora albicantia are generally made up almost exclusively of the 

 thickened elastic walls of the arteries which originally supplied the corpus 

 with blood, and which have subsequently been squashed together. 



In all Cetaceans, the corpus luteum continues to function throughout 

 pregnancy, but degenerates soon after the young is born so that during the 

 second half of the period of lactation only the corpus albicans remains. 

 After a few years, the latter disappears completely in terrestrial mammals, 

 and also in seals and sea-lions, so that, for instance in cattle, it is unlikely 

 that more than three corpora albicantia could be found at one time in 

 one and the same cow. However, in the few Odontocetes examined so far 

 (Dolphins, Pilot Whales) and in all Rorquals, it appears that the corpora 

 albicantia, while diminishing in size, never disappear completely. Thus in 

 Rorquals, where they start out with a brown colour and a diameter of 

 three to five and a half inches, the corpora fade and shrivel until finally 

 they have a diameter of a quarter to half an inch. By virtue of their 

 persistence, the corpora albicantia enable biologists to diagnose how often 

 ovulation has occurred in a given whale. Such diagnoses are impossible 



