2 THE VERTEBRATE SKELETON. 



palaeontology and embryology. Palaeontological evidence is 

 afforded by the fact that all the lower groups of vertebrates 

 Fish, Amphibia, and Reptiles had in former geological periods 

 a greater proportion of species protected by well-developed 

 dermal armour than is the case at present. Embryological 

 evidence tends the same way, inasmuch as dermal ossifications 

 appear much earlier in the developing animal than do the 

 ossifications in the endoskeleton. 



Skeletal structures may be derived from each of the three 

 germinal layers. Thus hairs and feathers are epiblastic in 

 origin, bones are mesoblastic, and the notochord is hypo- 

 blastic. 



The different types of skeletal structures may now be con- 

 sidered and classified more fully. 



A. EXOSKELETAL STRUCTURES. 



I. EPIBLASTIC (epidermal). 



Exoskeletal structures of epiblastic origin may be developed 

 on both the inner and outer surfaces of the Malpighian layer of 

 the epidermis 1 . Those developed on the outer surface include 

 hairs, feathers, scales, nails, beaks and tortoiseshell; and 

 are specially found in vertebrates higher than fishes. Those 

 developed on the inner surface of the Malpighian layer include 

 only the enamel of teeth and some kinds of scales. With 

 the exception of feathers, which are partly formed from the 

 horny layer, all these parts are mainly derived from the Mal- 

 pighian layer of the epidermis. 



1 The skin consists of an outer layer of epiblastic origin, the 

 epidermis, and an inner layer of mesoblastic origin, the dermis. The 

 epidermis is divided into two principal layers, an outer one, the horny 

 layer or stratum corneum, and an inner one, the stratum Malpighii. The 

 innermost part of the stratum corneum is distinguished as the stratum 

 lucidum, and the outermost part of the stratum Malpighii as the stratum 

 granulosum. 



