THE REPRODUCTIVE ORGANS 



407 



in length, the age or size of the whale evidently ceases to be the factor which dominates 

 the size of the testis, and other factors must be sought in order to explain the great 

 variations in its size in large whales. In the porpoise the testis is described by Meek 

 (1918) as undergoing an enormous development in the summer (northern hemisphere), 

 the breeding season being in July and August, and it is natural to suppose that some- 

 thing similar might occur in the Balaenopteridae. If this were so the size of the testis 

 might provide a valuable clue to the period and duration of the breeding season. 

 Unfortunately, however, there is no evidence that this is the case. If the testes of all 

 the sexually mature whales are plotted, according to the time of year and volume of 

 the testis, there is no indication of any correspondence between the two. The great 

 diversity in the size of the testis in different whales might be accounted for if there is 

 considerable individual variation and at the same time a slight increase in size in 

 answer to a stimulus associated with breeding. 



The most instructive observations on the testis are those made from the histological 

 point of view. One of the most striking features of the testis is the extraordinarily 

 small number of spermatozoa which are normally to be seen in sections, and it may be 

 said at once that the examination of sections gives no support to the supposition that 

 the testis might undergo any important increase in size when breeding takes place, 

 for the largest testes appear to contain no more spermatozoa than the smaller. 



Sections of the immature testis (Fig. 140) show small tubules, of which the wall 

 consists of a layer of small cells with small, 

 strongly staining nuclei. The lumen of each 

 tubule is completely filled by a comparatively 

 small number of large cells with large nuclei 

 which do not stain very strongly and of which 

 only about half a dozen appear in transverse 

 sections of tubules. There is often plenty of inter- 

 stitial tissue, but the spacing of the tubules varies 

 considerably. In immature Blue whales the 

 tubules appear often to be rather more tightly 

 packed and slightly larger than in Fin whales. 

 The histological appearances of the testis in the 

 foetus do not appear to differ in any way from 

 those of the large immature whale. 



At the approach of maturity the first indication 

 of a change is the appearance here and there of 

 division stages in the nuclei of the large cells in 

 the lumen of the tubules. After this the tubules 

 become greatly enlarged and various other changes 



take place. The general appearance of sections of the mature testis is subject to 

 considerable variation in any particular species, and this is only in part due to the 

 different degrees of freshness in which the material is fixed. For good fixation of 



Fig. 140. Section of testis tubules of 

 an immature Fin whale. 



