146 HISTORY OF THE HUMAN BODY 



cations, for use during active adult life, and are not empha- 

 sized in the embryonic history of higher forms. 



This stage of the chondro cranium, or the selachian stage, 

 as it may be called, is passed through with during the de- 

 velopment of all the higher vertebrates, and although in 

 the various forms the shape and proportion of the parts often 

 differ widely in anticipation of the various needs of the adult, 

 they all possess in common the origin in the same' way, from 

 the same elemental parts, and the characteristic regions may in 

 all cases be readily identified. 



For the next stage in this history it will not be necessary 

 to have recourse to embryology save to verify the conclu- 

 sions, since it is represented with almost diagrammatic clear- 

 ness among the ganoids, a very few of which have been, by 

 a fortunate chance, saved from the general destruction of the 

 Order during an earlier geological period. This stage may be 

 thus conveniently denominated the ganoid stage, for the type 

 of which we may select the sturgeon. Although similar to the 

 selachians in many respects, this animal differs markedly 

 from them in its external covering, for while the former is 

 evenly and uniformly covered by small placoid scales arranged 

 in a regularly imbricated pattern, the sturgeon possesses a 

 series of large, bony plates, or scutes, as they are called, 

 which may be considered as having been formed originally 

 from the fusion of the basal pieces of many scales. These 

 scutes are arranged on the body in longitudinal rows, leaving 

 the intervening regions bare, but are continued over the head 

 as somewhat modified scutes, the edges of which are in con- 

 tact, thus forming an external armor, with sutures between the 

 different scutes (Fig. 19, A). Immediately beneath this lies 

 a cartilaginous skull, very similar to that of selachians, and 

 the dermal armor encases it like an external skull, which it 

 really is. These dermal plates are quite definite in their 

 arrangement, and the same general plan may be followed 

 throughout the Order of ganoids. The snout, or rostrum, is 

 covered by a series of small rostral plates, which extend back 

 as far as the nostrils ; back of these openings may be found a 



