THE REPRODUCTIVE ORGANS 391 



The Structure of these corpora lutea is similar to that which the corpus luteum of 

 pregnancy assumes after parturition and the beginning of lactation, but they are 

 smaller and still more compact and tough. Sometimes little more is seen than a scar 

 at the apex of a hard and inconspicuous knob on the surface of the ovary. This, when 

 cut open, shows radial white connective tissue strands with a small amount of whitish 

 yellow tissue between them. Careful slicing of the ovaries reveals no traces of older 

 corpora lutea which are not to be distinguished on the surface. The staining with Nile 

 Blue is again faint and general. 



It will be seen that two quite different types of corpus luteum are to be found in 

 the ovaries of these whales. In the first place there is the functional corpus luteum of 

 ovulation or pregnancy which is a large and conspicuous structure composed mainly of 

 soft luteal cells. One cannot say how long the corpus luteum of ovulation (i.e. where 

 pregnancy does not supervene) remains unchanged, though this is presumably for 

 a comparatively short period. The corpus luteum of pregnancy persists as such only 

 until the end of gestation. In the second place there is the old functionless corpus luteum 

 formed by a kind of metamorphosis of the functional corpus luteum of ovulation or 

 pregnancy. This body remains essentially the same during its earlier stages in the 

 period of lactation and in its later stages of gradual absorption. The two types may for 

 purposes of convenience be referred to as corpus luteum a and corpus luteum b. 



There are several reasons for inferring that the retrogression of the corpus luteum 

 after gestation and lactation is extremely slow, so much so in fact that it is probably 

 never completely absorbed. The co-existence of a corpus luteum of pregnancy with 

 several of these corpora lutea b, some at least of which must have persisted since a 

 previous breeding season, is in itself evidence that this is the case. It is at least quite 

 certain that the corpus luteum persists for more than a year since, although there is 

 an annual breeding season among whales which falls only in a certain season, no mature 

 female is ever found, except those which have evidently only just become mature, 

 which has not several corpora lutea b in the ovaries no matter at what time of year 

 it is captured. A slight indication of the rate of absorption of the corpus luteum b 

 is shown in Figs. 127 and 128 in which are plotted the mean diameters of the largest 

 (and therefore presumably the most recent) corpus luteum b in either of the ovaries 

 where a corpus luteum of pregnancy was present. The plotted points show in general 

 in the case of Fin whales a gradual reduction in size of the corpus luteum b during 

 gestation, but there are insufficient data referring to the latter part of the period of 

 gestation to allow of any quantitative estimation of the average rate of regression. The 

 data in the case of Blue whales are insufficient to show any very definite results. 



In any pair of ovaries containing fairly numerous corpora lutea b, it is found that 

 the smaller corpora lutea are more numerous than the larger, and since the size is 

 a rough indication of the age, it follows that the older the corpus luteum the slower 

 becomes its rate of decrease in size. Fig. 128 suggests that in the case of Fin whales 

 the youngest corpus luteum b shrinks from about 5 cm. diameter to 3 or 4 cm. during 

 a period of about 10 months (i.e. the period of gestation). Thus one might say on a 



