64 HUMAN BIOLOGY 



matrix; in the early ganoid and dipnoan fishes the incom- 

 plete blocks or half-rings secreted in the elastic membrane 

 around the notochord may be seen in various stages of 

 development; in the oldest amphibians each vertebra was a 

 complex of eight pieces; and it is only in the higher verte- 

 brates that they become reduced in number. Meanwhile 

 the notochord loses its functional importance in the adult 

 but may always be identified in embryonic stages. 



THE ORIGIN OF JAWS AND TEETH 



The internal skeleton of the mouth and gill pouches in the 

 ostracoderms remained, so far as the material indicates, in 

 the purely cartilaginous stage, if indeed it was developed 

 at all. In the sharks these cartilaginous supports of the 

 mouth and gill arches became strengthened by the deposition 

 of calcium carbonate; but in the more direct Hne of forms 

 leading to the higher vertebrates the primary jaw cartilages 

 very early became overlaid by bony plates bearing teeth. 

 These teeth at first were nothing but minute thorns hke 

 those borne by the skin all over the body in certain ostraco- 

 derms and in modern sharks; but in and around the mouth 

 these dense bone-hke thorns speciahzed into true teeth; 

 meanwhile those on the surface of the body gave rise to the 

 enamel-hke scales, while those on the top and sides of the 

 head fused into the smooth skull and jaw plates of the early 

 ganoid fishes. 



Thus while the better known ostracoderms appear to have 

 been approximately ancestral to the modern Agnatha, or 

 so-called **jawless" cyclostomes, some remotely related 

 types of early chordates gave rise to the Gnathostomata, 

 or typically jaw-mouthed series of forms including the 

 sharks and their allies the crossopts, or lobe-fmned ganoids, 

 the actinopts, or true ganoids (ancestral to the modernized 

 teleost fishes) and finally the dipnoan, or double-breathing 

 fishes. 



AN IMPORTANT EXPERIMENT IN BREATHING 



Very early in the history of the lobe-fmned ganoid stock, 

 which seems to have lived in swampy streams subject to 

 occasional drying, a small accessory breathing organ was 



