THE CORPUS LUTEUM 353 



it is probably of the order of only 10-15 days ' (Eckstein, 1949, p. 400). In some mammals the corpora 

 lutea of ovulation are short-lived and probably non-functional, but they can be activated by a copula- 

 tion which does not result in pregnancy, or by mechanical stimulation of the cervix. They may then 

 persist longer and are called corpora lutea of pseudopregnancy. With the exception of some carnivores 

 and marsupials pseudopregnancy appears to be largely confined to the rodents. 



There is no evidence of pseudopregnancy in any of the cetaceans which have been studied and it is 

 unlikely that this condition occurs in whales, as it could hardly have passed undetected. It is here 

 assumed that the corpus luteum of ovulation in the whale conforms to the generalization made by 

 Eckstein (1949) and that it persists in a recognizable form for less than a month, probably for about 

 15-20 days. 



If fertilization and implantation occur the corpus luteum persists for a long period as a corpus 

 luteum of pregnancy. In the whale this period covers the duration of pregnancy, but in some mammals 

 the corpus luteum degenerates before the end of pregnancy, or may be replaced by a set of accessory 

 corpora lutea as in the mare and elephant (Amoroso, Hancock and Rowlands, 1948; Perry, 1953). 



Accessory corpora lutea are usually defined as corpora formed as the result of luteinization of an 

 unruptured follicle at the same time as the normal corpus forms, whether it be a corpus luteum of 

 ovulation, or of conception. In the mare during pregnancy a set of corpora are formed by the luteiniza- 

 tion of all or nearly all the follicles with antra; the larger follicles ovulate, the smaller luteinize 

 (Amoroso, Hancock and Rowlands, 1948). In the elephant the accessory corpora also form from 

 either ovulated or unovulated follicles (Perry, 1953). Accessory corpora lutea in the Norway rat form 

 by luteinization of unruptured follicles (Hall, 1952). In histology and function they appear to be 

 identical, whatever the mode of formation. Brambell (1956) uses the term accessory corpora lutea 

 as synonymous with corpora lutea atretica, for certain animals, but there is an important difference 

 which he points out earlier. The corpora lutea atretica form from medium and large unruptured 

 follicles by ' hypertrophy and hyperplasia of the cells of the theca interna after the degeneration of 

 the membrana granulosa'. The luteal tissue in true corpora lutea is derived from the membrana 

 granulosa. 



In the present account the term ' accessory corpus luteum ' is used to describe corpora formed at the 

 same time as a corpus luteum of pregnancy or a corpus luteum of ovulation, whether derived from 

 ovulated follicles or from unruptured follicles. In the case of a multiple ovulation when there are 

 only one or two foetuses and more than one or two of the corpora have rupture points, the largest 

 are assumed to be corpora lutea of conception and the others are classed as accessory corpora lutea 

 of pregnancy. Similarly when there is a multiple ovulation without conception, the largest ruptured 

 follicle is assumed to be the corpus luteum of ovulation and the others, whether ovulated or not, are 

 termed accessory corpora lutea of ovulation. In the fin whale there is no evidence for the formation of 

 accessory corpora lutea during pregnancy, as in the mare, elephant, viscacha, and porcupine (Amoroso, 

 Hancock and Rowlands, 1948; Perry, 1953; Pearson, 1949; Mossman and Judas, 1949), and this is 

 not a regular feature of pregnancy in the whale. In any case accessory corpora lutea only comprise 

 3-4% of all corpora lutea. 



Formation of the corpus luteum 



The humpback whale is the only species of baleen whale which has been extensively studied in the 

 breeding season and it is convenient to complete the fin-whale picture by reference to the findings of 

 Chittleborough (1954). First, it is necessary to show that the size of the fully formed corpus luteum 

 is similar in the two species ; it has already been established that the sizes of follicles at different stages 

 of the sexual cycle are similar. 



4-2 



