352 DISCOVERY REPORTS 



accepted that in mammals follicular activity is limited but not necessarily suspended during pregnancy, 

 because the secretion of progesterone by the corpus luteum prevents pre-ovulationary differentiation 

 and causes cystic atresia (Gillman, 1941 ; Van der Horst and Gillman, 1945, 1946). 



It is well known that in the rat there are oestrous cycles during pregnancy (Long and Evans, 1922; 

 Nelson, 1929; Swezy and Evans, 1930; Swezy, 1933). The follicular development proceeds only up to 

 a certain point, when cystic atresia begins, and corpus luteum cysts sometimes form, the granulosa 

 degenerating and the theca interna luteinizing. Similar luteinized cysts are occasionally found in 

 whale ovaries (p. 351). In the guinea-pig (Bujard, 1953) the percentages of growing follicles at different 

 stages of pregnancy are indicative of four oestrous cycles. In Elephantulus (Van der Horst and Gill- 

 man, 1945) there are three phases of follicular growth during pregnancy; in early pregnancy small 

 cystic follicles form, which are replaced by large cystic follicles and they in turn by small cystic 

 follicles. In some pinnipeds Harrison, Matthews and Roberts (1952) found two periods of follicular 

 stimulation after implantation of the blastocyst. In the elephant seal, Mirounga leonina, Laws (1956 c) 

 found one period of follicular growth during the free blastocyst stage and another follicle cycle just 

 after implantation. 



Williams, Carrigus, Norton and Nalbandov (1956) observed mating by ewes in early and late 

 pregnancy. Heats during pregnancy were not accompanied by ovulation in the animals which were 

 killed to check on this point. They found a significant increase in follicle number and follicle size 

 from early pregnancy up to the 25th day of pregnancy ; during the remainder of pregnancy the follicle 

 number remained constant, but follicle size decreased significantly. 



The follicular activity in the fin whale during pregnancy is at a maximum at mid-pregnancy which 

 is, for the average female, in November/December. Elsewhere (p. 450) it is shown that the fin whale 

 is probably seasonally monoestrous with peak ovulatory periods in June-July and November- 

 December. It seems probable that the follicular cycle during mid-pregnancy in the female fin whale 

 represents a suppressed oestrous cycle as for example in the rat, guinea-pig and sheep. 



In some mammals ovulation is of regular occurrence during pregnancy. In the mare, for instance, 

 the corpus luteum of pregnancy regresses after about 30 days and is replaced by a set of accessory 

 corpora lutea by the luteinization of all follicles with antra, the larger of which ovulate (Asdell, 1946; 

 Amoroso, Hancock and Rowlands, 1948). Similarly in the elephant, Loxodonta africana, further 

 corpora lutea are formed during pregnancy (Perry, 1953); in the rodent, Lagidium peruanum (Pearson, 

 1949) and in the porcupine, Erithizon (Mossman and Judas, 1949) accessory corpora lutea form during 

 pregnancy. Hansson (1947) showed that in mink (Mustela vison) oestrus, mating and ovula- 

 tion can occur during the free blastocyst period, and recently Harrison and Neal (1956) and Neal and 

 Harrison (1958) have shown that the badger (M. meles) may have up to 9 months delay in implantation 

 during which as many as three ovulations may take place. 



Although the fin whale has a follicular cycle during pregnancy there is no evidence that ovulation 

 ever occurs during pregnancy. There are a number of primiparous females in the present material 

 which have only ovulated once and many of these are in late pregnancy, in lactation, or post-lactation. 

 In these animals it is certain that no ovulations can have occurred during pregnancy. 



THE CORPUS LUTEUM 



Corpora lutea are normally formed from all ruptured follicles, but usually they soon degenerate if 

 fertilization of the ovum and implantation do not occur, and are then referred to as corpora lutea of 

 the cycle or corpora lutea of ovulation. The life-span of the corpus luteum of ovulation in different 

 species is remarkably uniform and independent of body size. ' The range of variability encountered is 

 about 10-20 days (the upper limit being represented by the cow), but in the great majority of animals 



