EXTERNAL CHARACTERS OF BLUE WHALES 315 



mouth leaving a broad, more or less fiat palate between. As the plates grow the basic 

 strips spread inwards until, when they are fully developed, no part of the palate remains 

 except a narrow ridge in the median line. This condition, however, is not reached until 

 long after the whale is born. Even when the foetus is ready for birth the longest plates 

 measure only about 2-5 to 3-0 cm. and are arranged along the outer edge of the jaw, 

 leaving a wide area of uncovered palate. 



The rate of growth of the baleen in relation to the rate of growth of the whale can 

 be examined in quite a simple manner by plotting the recorded lengths of the longest 

 plates against the length of the whale. There are unfortunately very few baleen measure- 

 ments for whales between 7-0 and 17-0 m., but there are just enough to indicate the 

 course taken by the curve of growth. In Fig. 49 all the records of the lengths of the 

 longest baleen plates are shown, and it will be seen that the plotted points may be 

 divided in a sense into two groups. The first of these consists of the points derived 

 from whales measuring 17-0 m. or more and forms a wide but regular series sloping 

 upwards from baleen lengths of about 40 cm. to lengths of 70 to 100 cm. for the largest 

 whales. This must mean that though there is some little individual variation in the 

 length of the baleen, the plates must grow by about 5 cm. for every increase of i m. 

 in the length of the whale. The second group consists of only a very few points derived 

 from foetuses and young whales of less than 17-0 m. These points fall into a compara- 

 tively well-defined line which indicates a rate of growth appreciably slower than in 

 the larger whales. The important feature of the graph, however, is that if a line is 

 drawn to represent the average slope of the plotted points, or in other words the mean 

 rate of growth of the baleen, the part built on the smaller group of points is not directly 

 continuous with the part built on the larger group, and one must conclude that there 

 is a sudden spurt in the growth of the plates during the whale's growth from about 

 16 to 18 m. The possibility arises that this is some functional development. The ques- 

 tion, however, will be dealt with when the growth of the calf is considered and it will 

 be shown that the sudden increase in size of the baleen plates is in fact almost certainly 

 associated with a change from a diet of milk to a diet of krill. 



As to other features of the chart we see that considerable individual variation occurs, 

 but that there is no particular grouping which might suggest any racial distinction. 

 Taken as a whole, the plotted points show that in general the length of the plates varies 

 fairly uniformly with the length of the whale. 



Observations on the numbers of baleen plates (excluding some subsidiary inner 

 plates which are not seen from the outside) show that they vary from about 250 to 400 

 on each side. A sufficiently thorough analysis of the records is given by the table of 

 frequencies given at top of page 316. 



This gives an idea of the numbers of baleen plates normally present in southern 

 Blue whales and the extent to which the numbers may vary. It also shows that 

 there is no significant difference in this respect between males and females or 

 between the Blue whales of South Georgia and South Africa. 



As regards the width of the baleen at the base only twelve measurements were made 



8-2 



