_two generations of teeth 



third generation tooth 



A B 



Figure 8-65. Section through tooth and jaw of alligator, A, showing second and third generation 

 teeth in relation to functional tooth. B, vertical section through fully formed tooth. 



by Kiikenthal, Rose, and others, supposes that even the tri- 

 conid tooth is a product of fusion of three simple teeth. In 

 view of the compound nature of the placoid scale of Sqttalus, 

 this seems most probable and would mean that all mam- 

 malian and most tetrapod teeth are in fact polymers. 



The teeth of mammals, at least the molariform teeth, 

 have an intricate pattern of cusps on them. This pattern, 

 which involves a complex terminology, is useful in the tax- 

 onomy of the group. The heterodonti of the mammal makes 

 possible the use of a dental formula (Chapter 3). This de- 

 vice indicates the numbers of the various types of teeth in 

 the upper and lower jaws. Such a formula can have a tax- 

 onomic value since it reflects an aspect of adaptive modi- 

 fication. 



RESUME 



In terms of the skin and its derivatives, there are a few 

 salient points. The stratified epidermis of the vertebrate sep- 

 arates this group from the protochordates and invertebrates 

 in general. The histology of the skin has not been used in 

 subdividing the vertebrates. It can be noted that the epider- 

 mis of the cyclostomes is thicker in terms of the number of 

 cell layers than that of gnathostome and that the dermis is 

 perhaps less complex in its structure. These differences can- 

 not be sharply defined, however, since most of the agnaths 

 are extinct and the structure of their skin is unknown. 



Among the gnathostomes, the amphibians are distinct in 

 terms of their saccular or simple, alveolar, mucous and poison 

 glands. The mammals in contrast have tubular, very coiled 

 sweat glands. The general picture is much as before with 

 three lines of tetrapods (amphibian, reptile, and mammal) 

 stemming from some unknown ancestor. Neither the amphi- 

 bian, with its specialized glands, nor the reptile, with its 



stratum corneum modified into scales, can be considered 

 the more primitive. 



In fishes multicellular epithelial glands are either lacking 

 or are few in number and highly specialized. The exception 

 is the line of large mucoid glands in the myxinoid. In place 

 of the multicellular glands, fishes have large numbers of 

 mucoid cells which pour their secretion on to the surface of 

 the skin and then are replaced from below. Other kinds of 

 secretory cells are usually present. These may be very large 

 and sunk deep in the epidermis. The nature of their se- 

 cretions is not known, but in some cases they appear to pro- 

 duce the cementing matrix of the epidermis itself Usually 

 they contribute to the surface slime layer along with the 

 mucous cells. 



Although much is known about the fine structure of the 

 scales of vertebrates and although scales form an important 

 part of the fossil record, there is as yet no clear cut picture 

 of the evolution of these structures. The histology of agnath 

 plates is in some cases like that of gnathostomes or quite 

 different as in the case of the aspidin. The basic differences 

 in scale and armor structure appear to lie in their develop- 

 ment; however, such information is still lacking for many 

 types. 



For teeth, the same situation exists as for scales. The 

 agnaths are defined as lacking marginal teeth on the jaws 

 although they possessed the dermal denticles from which such 

 teeth are presumed to have evolved. Within the vertebrates, 

 teeth are fairly uniform in structure with highly modified 

 types cropping up in several groups: sharks, dipnoans, and 

 the various tetrapods. A careful histological study of the 

 teeth involved may confirm the proposed relationship be- 

 tween the holocephalans and the bradyodont sharks. Other, 

 and similar, problems may also be solved through such 

 comparative studies. 



252 • THE SKIN AND ITS DERIVATIVES 



