358 CETACEANS. 



every tube has a minutely wavy course. The cemental tubuli are 

 rather smaller and give oiF more numerous lateral branches than 

 the dentinal tubuli ; these branches everywhere anastomose ex- 

 tensively with the radiated tubes of the calcigerous cells. The 

 cemental tubes are rather less and the cells larger and more elongated 

 in the cement of the Platanista than in that of the Cachalot. 



145. Development. — The primitive seat of development of the 

 tooth-matrix in the vascular membrane or gum, lining an open 

 groove on the alveolar border of the maxillary bones, is maintained 

 much longer in the Cetacea than in the higher organized Mammalia ; 

 a greater proportion of the tooth is, also, developed before the 

 matrix sinks into or is surrounded by a bony alveolus, and, with the 

 exception of the rudimental tusks in the Narwhal, is at no period 

 entirely inclosed in a bony cell ; in which respect the Cetacea 

 offer an interesting analogy to true fishes. In a preparation of the 

 jaw of a young Porpoise(l), which I added to the Hunterian 

 Series of Comparative Anatomy in 1832, the half-formed posterior 

 teeth are shown imbedded in the gum only, the growth of the jaw not 

 having yet attained their level. Hunter, whom none of the pecu- 

 liarities of the Cetaceous organization had escaped, was aware of 

 this circumstance in their dental development; but, not having 

 carried his researches on this subject into the earlier periods of 

 foetal life in man and mammalia, when the aid of a microscope 

 is needed, he believed the phenomena of the papillary and open 

 follicular stages to be peculiar to the Cetacea ; thus he says, 

 " The situation of the teeth, when first formed and their progress 

 afterwards, as far as I have been able to observe, is very different 

 in common from those of the quadruped. In the quadruped the 

 teeth are formed in the jaw, almost surrounded by the alveoh, and 

 rise in the jaw as they increase in length, the covering of the 

 alveoli being absorbed. The alveoli afterwards rise with the teeth 

 covering the whole fang ; but in this" (the whale-) " tribe the teeth 

 appear to form in the gum upon the edge of the jaw, and they 

 either sink in the jaw as they lengthen, or the alveoli rise to inclose 

 them ; this last is most probable, since the depth of the jaw is 



(1) No. 325 A. 



