vii THE EXTERNAL FEATURES OF THE BODY 431 



much the same arrangement in early stages as will be seen in the 

 Lung-fishes (Fig. 200, A), the dorsal part of the embryonic body being 

 curved round the periphery of the egg. As the embryo increases in 

 length the growth of the posterior end is specially active and the 

 general proportions become very much as in Polypterus. The some- 

 what tadpole-like appearance of the larva, caused by the persistent 

 spherical shape of the main mass of yolk, is again apparent — 

 especially in Amia and Lepidosteus (Fig. 198, A). As in Polypterus 



. — Stages in the development of Lepidosteus. 



a, anus ; b.c, widely gaping buccal cavity ; c.o, cement organ ; op, operculum ; p.f, pectoral fin ; 



pl.f, pelvic jin. 



the intestinal portion of the alimentary canal rudiment is relatively 

 slender in form, arising by a process of actual backgrowtb of the 

 posterior trunk region rather than by gradual modelling of the yolk 

 as is the thick intestinal rudiment in the Lung-fish. Conspicuous 

 characteristics of the actinopterygian Ganoid larvae are the presence 

 of well-developed cement-organs and the absence of external gills. 



In the Teleostean fishes there has come about with the high 

 development of telolecithality a great reduction in the angular extent 

 of the embryonic rudiment during its early stages. Consequently 

 there is very slight ventral curvature round the yolk. In the 



