278 ANATOMY OF VERTEBRATES. 



modification of the dental system is especially adapted. The 

 baleen-pulp is situated in a cavity at the base of the plate, like 

 the pulp of a true tooth ; whilst the external cementing material 

 maintains, both with respect to this pulp and to the portion of the 

 baleen-plate which it develops, the same relations as the dental 

 capsule bears to the tooth. According to these analogies, it 

 must follow, that only the central fibrous or tubular portion of 

 the baleen-plate is formed, like the dentine, by the basal pulp, 

 and that the base of the plate is not only fixed in its place by the 

 cementing substance or capsule, but must also^receive an acces- 

 sion of horny material from it answering to the cement of true teeth. 



In Baloena mysticetus there are about 200 large marginal 

 plates on each side, from 10 to 14, rarely 15, feet in length, 

 and about 1 foot in breadth at their base ; these plates are 

 overlapped and concealed by the under lip when the mouth is 

 shut. In the Balanopterce or fin-backed whales, figs. 217, 218, 

 the baleen-processes, e, internal to the marginal plates, are fewer 

 and smaller than in the Balance ; the marginal plates, c, are more 

 numerous, exceeding 300 on each side ; they are broader in pro- 

 portion to their length, and much smaller in proportion to the 

 entire animal ; they are also more bent in the direction transverse 

 to their long axis. 



A thin transverse section of baleen, viewed with a low mag- 

 nifying power, demonstrates that the coarse fibres, as they seem 

 to the naked eye, which form the central substance, are hollow 

 tubes with concentric laminated walls. When a high magnifying 

 power is applied to such a section, the concentric lines are shown 

 not to be uniform, but interrupted here and there by minute 

 elliptical dilatations, which are commonly more opaque than the 

 surrounding; substance, and which, like the radiated cells of true 



o * * 



bone, are probably remains of the primitive cells of the formative 

 substance ; similar long elliptical opaque bodies or cells are dis- 

 persed irregularly through the straight parallel fibres of the dense 

 outer laminae of the baleen-plate. The chemical basis of baleen 

 is albumen hardened by a small proportion of phosphate of lime. 



The Bal&nida, before they acquire their peculiar array of 

 baleen-plates, manifest in their foetal a^e a transitory condition 



J. O v 



of a true dental system, abortive and functionless, but homologous 

 with that which is normal and persistent in the majority of the 

 order. In an open groove which extends along the alveolar border 

 of both the upper and the lower jaws, there is a series of minute, 

 conical, acute or obtuse, single or double, denticles, fig. 219, with 



