SOME INTERNAL STRUCTURES 45 



pieces composing the sternum is reduced. In Meso- 

 plodon there may be only four, and in the Sperm 

 whale there are but three pieces ; moreover, in this 

 latter whale the double character of the sternum 

 is especially obvious. Two of the three pieces out 

 of which it is composed are paired bones, while the 

 last shows some indications of a longitudinal division 

 into two. A further shortening of the sternum is 



o 



exhibited in the Cachalot by the fact that there are 

 only four ribs which reach it. 



These three types of Cetaceans seem to show that 

 there has been a progressive shortening of the 

 sternum. But the facts are not, it is hardly necessary 

 to point out, conclusive as a demonstration of this 

 probability. More certain evidence is afforded by 

 the actual stages of development of the breast-bone 

 of the common Porpoise. In this whale the actual 

 proportions of the sternum during growth to the 

 adult condition have been found to lessen in a 

 marked fashion, which leaves no doubt that here 

 at least the sternum is a part of the skeleton which 

 is shrinking. 



The extreme of the shrinkage of the sternum is 

 realised in the whalebone whales, in which we have 

 seen, and shall see, so many grounds for regarding 

 as in many respects the most modified of whales. In 

 these animals the sternum is reduced to a single 

 piece, which is heart-shaped in the Balcsna australis, 

 and sometimes cross-shaped in the Rorquals ; more 

 generally it has in these latter Cetaceans the form 

 of a T. With the sternum in these whales articu- 



