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358 CETACEANS.

every tube has a minutely wavy course. The cemental tuhuli arerather smaller and give off more numerous lateral branches thanthe dentinal tubuli; these branches everywhere anastomose ex-tensively with the radiated tubes of the calcigerous cells. Thecemental tubes are rather less and the cells larger and more elongatedin the cement of the Platanista than in that of the Cachalot.

145. Development .—The primitive seat of development of thetooth-matrix in the vascular membrane or gum, lining an opengroove on the alveolar border of the maxillary bones, is maintainedmuch longer in the Cetacea than in the higher organized Mammalia ;a greater proportion of the tooth is, also, developed before thematrix sinks into or is surrounded by a bony alveolus, and, with theexception of the rudimental tusks in the Narwhal , is at no periodentirely inclosed in a bony cell ; in w 7 hich respect the Cetacea offer an interesting analogy to true fishes. In a preparation of thejaw of a young Porpoise(l), which I added to the HunterianSeries of Comparative Anatomy in 1832, the half-formed posteriorteeth are shown imbedded in the gum only, the growth of the jaw nothaving yet attained their level. Hunter, whom none of the pecu-liarities of the Cetaceous organization had escaped, was aware ofthis circumstance in their dental development; but, not havingcarried his researches on this subject into the earlier periods offoetal life in man and mammalia, w r hen the aid of a microscopeis needed, he believed the phenomena of the papillary and openfollicular stages to be peculiar to the Cetacea ; thus he says,“ The situation of the teeth, when first formed and their progressafterwards, as far as I have been able to observe, is very differentin common from those of the quadruped. In the quadruped theteeth are formed in the jaw, almost surrounded by the alveoli, andrise in the jaw as they increase in length, the covering of thealveoli being absorbed. The alveoli afterwards rise with the teethcovering the whole fang ; but in this” (the whale-) “ tribe the teethappear to form in the gum upon the edge of the jaw, and theyeither sink in the jaw as they lengthen, or the alveoli rise to inclosethem; this last is most probable, since the depth of the jaw is

(1) No. 325 A.