74 WHALES 



Figure jj. Right lower jaw with tooth buds of a 4 foot Fin Whale foetus. 



During subsequent embryonic development the tail of the former 

 increases in I'elative size, while that of the latter decreases. This may well 

 be connected with a characteristic distinction between Mysticetes and 

 Archaeocetes on the one hand, and Odontocetes on the other: the course 

 the spinal arteries take along the various vertebrae. Beneath the vertebral 

 column lies the aorta, which continues as the caudal artery in the tail 

 (Fig. 95) . From the aorta, small spinal arteries connected with the blood- 

 v-essels in the vertebral canal branch off at every vertebra. Now the 

 skeleton shows clear traces of these spinal arteries, since they penetrate 

 the vertebrae to leave a slight groove. In the Odontocetes, these grooves 

 run behind the transverse spinal processes of the lumbar and anterior 

 caudal regions, and in the posterior caudal region they penetrate the 

 processes from the rear (Fig. 37). In Archaeocetes and Mysticetes, the 

 arteries also run behind the transverse processes in most of the lumbar 

 area, but in the posterior lumbar region they change course so that, in 

 the anterior caudal region, they run in front of the transverse processes 

 and penetrate them from the front (Fig. 38). The difference can be 

 explained by assuming that during embryonic development the aorta 

 and the caudal artery on the one hand, and the vertebral column on the 

 other, grow at different rates. Since in Mysticetes the tail grows faster than 

 the trunk, and in Odontocetes the trunk grows faster than the tail, we may 

 take it that these differences give rise to the differences in the arterial 

 grooves. 



We have already seen that the beginnings of the Mysticetes go as far 

 back as the middle Oligocene, about 27 million years ago. From the 

 Oligocene and Miocene, i.e. roughly 27-7 million years ago, fossil remains 

 of a primitive group of Mysticetes, the Cetotheriidae, have been discovered 

 in various parts of the world. The arrangement of their skulls and other 

 characteristics is such that they are considered to be the original ancestors 

 of the recent Mysticetes. They were, however, strikingly smaller (9-33 

 feet j . 



From the Pliocene (7-1 million years ago), a number of species are 



