THE ORIGIN OF THE DENTAL PLATES. 119 



might accordingly regard the anterior plates as premaxillary or vomerine, although in the latter 

 regard, i. e., re premaxillary and vomerine elements, we query whether we can justly introduce 

 this comparison in the holocephali. On the other hand, if the comparison be a legitimate one, we 

 might even go farther and regard the more median ridge of the large plates of the mouth-roof 

 as equivalent to the fused vomerine teeth, and look upon the remainder of these plates as having 

 arisen from fusion of the elements in a double row of palatine teeth. Of course, however, such 

 an interpretation would be purely hypothetical. 



One is inclined to look upon the anlage of a dental plate as the product of a single and 

 enormously enlarged dental papilla, circumscribed by a dermal fold, the induplicature of which is 

 deepest at the posterior margin of the papilla. The first deposition of hard material begins at the 

 outer surface of the papilla, and takes the form of a thin cap of dentine, soon, however, the tooth- 

 substance appears below at the points where the plate is to come in contact with the cartilage of 

 the head. And almost at the same time trabeculae and lamellae appear between, i. e., in the 

 substance of the plate, and produce a meshwork of spongy tooth-substance (pulp-dentine). The 

 mode of origin of the plate resembles closely that of bone when derived from connective tissue 

 (e. g., in Sphenodon). The mesenchyme cells in the papilla are collected together closely 

 at certain points and become transformed into odontoblasts, and from these, peripherally, the 

 dentine takes its origin. It may be remarked that the dentine is sometimes laid down in an 

 irregular way, with branching processes, its canals ramifying, unlike the parallel canals of true 

 dentine. Occasionally trabecules of the dental mass, especially in older individuals, show a 

 somewhat lamellar structure, and those which are first differentiated, that is, those lying inner- 

 most, are distinguishable from the later lamellae by their capacity to become stained. As 

 already noted, the entire dental plate is finally formed of a meshwork of dentine-like material, 

 whose trabecules thicken with age, so that finally the plate attains a high degree of hardness. 

 The spaces between the meshwork represent collectively a large, greatly branched pulp cavity, 

 whose cells in part have retained their former reticular arrangement, in part have become odonto- 

 blasts, as far at least as they become opposed to the trabecules. In the various ramifications 

 of the pulp cavity blood-vessels are often present. Enamel is not deposited ; nevertheless the 

 epidermis cells must have a certain influence on the character of the dentine, since the dentine 

 becomes glassy in character when in contact with the epidermis, but remains unchanged when- 

 ever the epidermis is lacking. The dental plates are fastened to the head cartilage by means of 

 a firm layer of connective tissue, which indeed here and there may enter the substance of the 

 plate, and for still stronger attachment claw-like outgrowths arise from the base of the plate, 

 especially from its anterior and lateral portions. 



Finally, I must refer to the presence of remarkable structures in the dental plates, which occur 

 only within the ridges above referred to. These take the form of a chalky mass, which appears 

 in cleared preparations and can be traced throughout the entire length of a dental ridge ; it 

 is partly inclosed within the meshes of the trabecules of the dentine, and by these partly 

 again broken up into rounded masses and processes. In transverse section this chalky mass 

 presents the appearance of a section of a many-rooted tooth, while in longitudinal section its 

 substance appears continuous, although greatly fenestrated. A more detailed examination 

 shows that we are here dealing with an especial variety of dentine ; that is, differentiated from 

 odontoblast-like embryonic cells, whose processes grow deeply down and develop canals which 

 from their parallel arrangement recall strikingly those of typical dentine. In any event, the 

 material in question can more accurately be designated as dentine than can the remaining spongy 

 substance of the dental plate. From the latter it is also distinguished in remaining colorless 

 after treatment with the usual stains for bone, and especially in retaining permanently, even in 

 the grown Callorhynchus, its soft and uncalcified condition. It may be noted that this soft 

 dentine is not present in the youngest embryonic stages ; it appears shortly after the caudal ends 

 of the plate are established and extends gradually from a hindward into a more anterior position. 



It has nothing to do with the origin of the hard structures of the plate, since it appears after 

 these have been laid down. It usually appears somewhat deeper than the outer surface of the 

 plate ; later it often comes to lie in close contact with it, and even extends thence inward, not 

 infrequently coming to be associated with the remaining meshwork of the dentine. What the 

 significance of this structure is remains in any event doubtful, and only with reserve do I express 

 the opinion that these soft masses of dentine represent the rudiments of former rows of single or 



