LENGTH AS A MEASURE OF AGE 409 



there were many in the intermediate stage, and Flower's statement that ankylosis begins 

 at both ends of the column was fully confirmed. 



No epiphyses were found on the articular surfaces of the atlas, and only a reduced 

 slip of bone on the anterior surface of the axis in one immature specimen. Otherwise 

 the epiphyses could usually be traced from the posterior surface of the axis throughout 

 the column, except sometimes in the extreme caudal vertebrae. 



In the most frequently noted intermediate stage the epiphyses were ankylosed from 

 the anterior end of the column to the ist thoracic, and from the posterior end forward 

 to the 7th or 8th caudal. In one or two specimens anterior ankylosis appeared to have 

 reached as far back as the 4th thoracic, while posterior ankylosis had not proceeded 

 beyond the ist caudal. No later stages were found, perhaps because of the difficulty of 

 differentiating between the last stages of ankylosis in dried specimens. 



We can state, then, that ankylosis commences at both ends of the column and is 

 completed among the anterior thoracic vertebrae. When the epiphyses of these vertebrae 

 are ankylosed, physical maturity has been attained and growth in length ceases. 



UNRELIABILITY OF LENGTH AS A MEASURE OF AGE 



In the course of the work on whales it has become very evident that length is an 

 uncertain guide to age — that is, relative age — except possibly as an average. This is so 

 even among immature whales, for in these the limit of length overlaps by more than 

 a metre the length of the smallest sexually mature whale. This can be seen in Fig. i. 

 The smallest sexually mature female Fin whale taken in season 1929-30 was 1875 m. 

 (No. 2630). It was pregnant. The largest immature female Fin was 21-4 m. (No. 3175). 



As might be expected, the lengths at physical maturity show at least equal divergence. 

 A female whale 21-05 m. long (No. 2915), and several very little longer, were physically 

 mature, but females more than 23 m. long have been examined in which the epiphyses 

 were still unfused. 



AGE EVIDENCE FROM THE CORPORA LUTEA 



Before attempting to discuss the correlation of physical maturity with the age data 

 from the corpora lutea, the origin and fate of the latter must be mentioned and the 

 reasons given for considering that they form an index of age. Part of this work has 

 been put forward in the report on Southern Blue and Fin whales (1929), but further 

 observations have led to a modification of the theory then suggested. 



On the expulsion of the ovum the Graafian follicle grows to form a corpus luteum. 

 If fertilization of the ovum follows, the corpus luteum persists practically unchanged, 

 in size at least, until late in gestation. At the end of gestation, or during lactation, it 

 shrinks and grows harder by the reabsorption of the luteal cells and the increasing 

 growth of connective tissue. If, on the other hand, the ovum is not fertilized, the 

 corpus luteum, which is formed exactly as in pregnancy, persists as a functional body 

 for a shorter period. There is no evidence concerning the shrinkage of the corpus luteum 



