v THE SKELETON 337 



inner and outer surfaces, that formed on the outer surface being the 

 enamel or ganoine already referred to. In the highly evolved Teleost, 

 where the scale has increased in area at the expense of thickness, 

 the addition of new bone on the flat inner and outer surfaces of the 

 scale is relatively small in amount as compared with that round 

 the edges. 



In accordance with variations from time to time in the metabolic 

 activity concerned in the production of the new bone the latter 

 tends to show variations in rapidity of growth, density and other 

 characters, and consequently to show a more or less distinct layered 

 arrangement. Where there are periodic variations in the metabolism 

 of the fish — associated it may be with sexual activity or with food 

 supply or with changes in the physical environment (e.g. seasonal 

 changes of temperature) — these variations may be duly chronicled 

 in the contemporary layers of the scales. Such scale records are 

 often particularly distinct and easily observed in the scales of the 

 Teleostei owing to their thin flat character and the predominance of 

 growth at their edges. 



The development of the Cycloid scales of Dipnoi has not been 

 investigated in detail. So far as the main features of their develop- 

 ment are concerned they apparently resemble the scales of Teleosts. 

 Like them they are, except at their posterior edge, deeply embedded in 

 the dermis. On their outer surface they are prolonged into numerous, 

 often recurved, spines which in all probability represent true 

 denticles although they have lost their primitive relation to the 

 epidermis. 



Bony Vertebral Column. — In all gnathostomatous Vertebrates, 

 except the Elasmobranchs (including Holocephali) and Sturgeons, 

 the vertebral column becomes in great part bony. The process of 

 ossification is found in its first beginnings in the Lung-fishes, where 

 the arches become ensheathed in bone. 



In the bony Ganoids and Teleosts ossification usually commences 

 in the connective tissue bounding the surface of the arcualia, the 

 first shreds of bone being completely cell-less. From the arches the 

 bone spreads over the surface of the chordal sheath (in Amia it 

 develops here first — Schauinsland) to form the rudiment of the bony 

 centrum. In Ooregonus it is stated (Albrecht, 1902) that for a time 

 two bony rings can be distinguished round each centrum (cf. variation 

 in Amia mentioned below on p. 339). From the thin superficial sheath 

 of bone an irregular spongework of bony trabeculae spreads outwards 

 and forms the bulky centrum of the definitive vertebra. As this 

 process goes on the basal portions of the cartilaginous arches become 

 surrounded by bone and may persist as four tracts of cartilage 

 running outwards through the bony centrum (e.g. Esox — Pike). 

 Most usually the arches become completely bony: the original 

 bony sheath covering their surface becomes perforated on its median 

 side by invading vascular connective tissue which destroys the 

 cartilage and deposits bone in its place. 



VOL. II z 



