THE REPRODUCTIVE ORGANS 389 



in pregnant ovaries are those that would later have discharged their ova had not 

 fertilization occurred. During lactation the larger follicles, having apparently attained 

 a size that is beyond retrogression, remain large but lose the turgidity they had before 

 and during pregnancy, while the smaller follicles retrogress to become again hidden 

 beneath the surface of the ovary. 



{d) The Corpus Liiteum of Ovulation 



In certain whales no foetus was found in the uterus yet a corpus luteum similar to 



the corpus luteum of pregnancy was present in the ovaries. Assuming that no foetus 



was missed in these cases, it may be said that these were corpora lutea of ovulation, 



i.e. representing an ovum which had been shed quite recently. There are, in such 



circumstances, two occasions on which a foetus may be missed. It may be lost at sea 



through premature birth when the whale is killed, or it may be so minute that it cannot 



be found. The former could hardly occur except when the foetus is fully large enough 



to leave unmistakable evidence of its presence in the uterus. There have in fact been 



two clear cases in which it has occurred, for in No. 373 (Fin) and No. 1602 (Sei), 



although no foetus was present, some of the membranes were still in the uterus, and 



even had these been lost the swelling and congestion of one cornu of the uterus could 



not have been missed. With regard to the second possibility it may be said that the 



smallest foetus can hardly be missed if searched for in the proper manner. When a 



functional corpus luteum is present in the ovaries the uterus is at once spread out and 



slit open from end to end. The foetal membranes of even a 2 mm. embryo form an 



object about the size of a thrush's egg and can readily be seen. It may be said then 



that those functional corpora lutea which were not found to be accompanied by a 



foetus, were corpora lutea of ovulation or were accompanied by a foetus not exceedmg 



I or 2 mm. in length. It is in any case certain that in all the whales in question 



ovulation had occurred relatively very recently. Of these whales there were nine Blue 



and four Fin whales. None had any sign of a foetus and the copora lutea were on the 



average smaller than the corpus luteum of pregnancy except in No. 250 (Blue) in 



which the corpus luteum had a mean diameter of 147 cm. This contained an enormous 



cavity and was obviously a very young structure. 



Corpora lutea of ovulation in Fin whales occurred at South Georgia once in February 

 among lifty-two mature females, and at Saldanha Bay once in June (the only adult 

 female), once in July (also the only adult female) and once in September (among three 

 mature females). Very small embryos were found twice at South Georgia in January 

 (one in 1926 and one in 1927) and once at Saldanha Bay in the only mature female 

 taken in August. Now only 8 per cent of the female Fin whales taken at Saldanha Bay 

 were adult, whereas at South Georgia over 60 per cent were adult. Thus the ratio of 

 ovulating females to other mature females is overwhelmingly greater at Saldanha Bay 

 than at South Georgia. That is to say, a far greater percentage of Fin whales are 

 ovulating during the southern winter than during the southern summer. 



Among Blue whales again at South Georgia one corpus luteum of ovulation was 



