368 DISCOVERY REPORTS 



pigment so that the colour of the lobes darkens with age. This progressive invasion by blood vessels 

 and phagocytes is shown in Text-figs. 13 A, B, and C, Text-fig. 14 and in Pis. V-VII. 



In the 'medium' corpora the smaller spiral vessels are more numerous in comparison with the 

 ' young ' corpus, and often impart a reddish hue to the pigmented layer so that bodies of this group 

 are usually a darker reddish brown in colour as opposed to the tawny brown, or even yellow colour of 

 the most recent corpora albicantia. 



'Old' corpora 



Evidence shortly to be presented establishes that this 

 group represents the final stages of regression of whale cor- 

 pora lutea, and within this group there is no further 

 reduction in size. The mean diameter of 2339 ' old ' corpora 

 albicantia is 2-013 ±0-025 cm -> corresponding to a weight 

 of about 5 g., and the size range is from 0-7 to 5-0 cm. 



They are characterized by increased centripetal penetra- 

 tion of thick -walled blood vessels and closing of the lumen 

 of vessels near the periphery so that the external white 

 connective tissue trabeculae become even more conspicuous. 

 The layer of pigmented tissue is further reduced and 

 concentrated so that it forms a thin hyaline pigmented zone 

 separating the inner, avascular, connective tissue from the 

 outer trabeculae which are now much thicker and composed 

 largely of contorted non-functional thick-walled blood 

 vessels (PI. VII, fig. 1). Text-fig. 13 C shows the macroscopic 

 appearance of one of the least regressed corpora in this 

 group, which still has a conspicuous amount of pigmented 

 hyaline tissue. Text-fig. 13 C, Text-fig. 14 and Pis. V-VII 

 show how the pigmented tissue is replaced by blood vessels 

 growing centripetally. 



There is no further reduction in the volume of the corpus albicans which is now composed largely 

 of unpigmented collagen, and further regression takes the form of progressive vascular and phagocytic 

 invasion and continuing reduction of the area of the pigmented zone. As this becomes thinner it 

 becomes increasingly more densely pigmented and the final stages of regression which have been 

 observed show in section an almost white, avascular, connective tissue body with a very thin darkly 

 pigmented zone outlining the internal septa. In places this pigmented zone may be completely 

 resorbed, as in PI. VI, figs. 4-7. The reduction and concentration of the pigmented area and its 

 replacement by sclerotic arteries is illustrated in Pis. V and VI. 



The rate of thickening of the walls of the blood vessels during regression of the corpus albicans is 

 shown in Text-fig. 15. For each of 10 'old', 12 'young' and 'medium' corpora albicantia and one 

 corpus luteum measurements of the external and internal diameters of the arterioles were made with 

 a projection microscope. Only data from stained sections of corpora in which it was possible to 

 measure 10-20 independent arterioles have been used, which explains the small size of the sample. 

 The mean percentage thicknesses of the vessel walls for the three groups of corpora are plotted against 

 the mean corpus diameter in centimetres, and it appears that the rate of the regression in size of the 

 corpora albicantia is correlated with the rate of change in the thickness of the vessel walls. Once the 

 corpora enter the ' old ' group their volume does not decrease further, but it is likely that the vessels 



I mm. 



Text-fig. 14. Thick cleared section of a typical 

 'old' corpus albicans. Note avascular core and 

 internal septa below. 



