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46 THE ORIGIN OF VERTEBRATES 



looked upon as similar to that of the pulp and dentine canals of 

 many fish-scales. 



( )n the other hand, this dermal covering of Pteraspis has heen 

 compared by Patten with the arrangement of the chitinous structure 

 of certain parts of the external covering of Limulus, a comparison 

 which to my mind presents a great difficulty. The chitin-layers in 

 Limulus are external to the epidermal cells, being formed by them ; 

 the layers in Pteraspis which look like chitin must have been interim! 

 to the epidermal layer; for each vascular canal which passes from 

 a pulp-cavity on its way to be distributed into the dentine canals 

 of the ridge gives off short side branches, which open directly 

 into the groove between the ridges. If these canals were filled with 

 blood they could not possibly open directly into the open grooves 

 between the ridges ; these openings must, therefore, have been covered 

 over with an epithelial layer which covered over the surface of the 

 animal, and consequently the chitin-like structure must have been 

 internal to the epidermis, and not external, as on Patten's view. 

 The comparison of this structure with the dentine of fish-scales 

 signifies the same tiling, for in the latter the epidermis is external 

 to the dentine-plates, the hard skeletal structure is in the position 

 of the cutis, not of the cuticle. 



The position appears to me to be this : the dermal cranial skele- 

 ton of vertebrates, whether it takes the form of a bonv skull or of 

 the dorsal plates of a cephalaspid or a pteraspid is, in all cases, not 

 cuticular, i.e. is not an external formation of the epidermal cells, but 

 is formed in tissue of the nature of connective tissue underlying the 

 epidermis. On the contrary, the hard part of the head-carapace of 

 the pateostracan is an external formation of the epidermal cells. 



If, then, this tissue of Pteraspis is not to be looked upon as 

 chitin, how can we imagine its formation ? It is certainly not bone, 

 for there are no bone-corpuscles ; it is a very regular laminated 

 structure resembling in appearance chitin rather than anything else. 



As in all cases of difficulty, turn to Ammocoetes and let us see 

 what clue there is to be found there. The skin of Ammocoetes is 

 peculiar among vertebrates in many respects. It consists of a number 

 of epidermal cells, as in Pig. 140, the varying function of which 

 need not be considered here, covered over with a cuticular layer 

 which is extraordinarily thick for the cuticle of a vertebrate skin ; this 

 cuticular layer is perforated with fine canaliculi, through which the 



