Od tlie larval development of Amia calva. 651 



the beginnings of the opercular series of dermal bones have appearcd. 

 The sucking disc is greatly reduced, altliough it does not in fact 

 disappear entirely (histologically) for several wecks; it remains as a 

 small flattened päd, scarcely to be noticed in the profile of the head, 

 althoiigh its distal surface, PI. 10, Fig. 19 b, still shows slight de- 

 pressions. Its atrophy takes place first proximally, later marginally; 

 the cells of its deepest tissue become greatly vacuolated and form a 

 sponge-like mass, and the cell wall which here forms its outer 

 boundary, gradually encroaches; the cells of the centro-distal region 

 are the last to retaiu their early character. The anus appears in the 

 now widely separated gap between the fin-folds of the abdominal and 

 anal regions, Muscle plates are to be traced ventralward on the ab- 

 dominal walls. The advances in the development of the uns include: 

 the growth of the pectorals, the bases of which, changing from their 

 crossopterygian character, are becoming less distinctly lobate as the 

 dermal rays make their appearance; the origin of the ventral fins as 

 minute dermal folds, arising independently, however, of the unpaired 

 abdominal or anal ; the appearance in the unpaired fins of supporting 

 Clements; these, the homologues of the radial cartilaginous fin Sup- 

 ports of WiEDERSHEiM, takc thcir origin along the dorsal body wall 

 in a continuous series, most prominent in the hindmost region and 

 on the ventral side in those portions of the unpaired fin which will 

 later become constricted ofi" as anal and caudal fins ; of these the 

 outlines will be seen to be now suggested. These supporting Clements 

 make their appearance as in the Teleosts some time before dermal 

 rays are to be determined, the latter when they come to arise seem- 

 ing to bud out from their distal ends, spreading out like taperiug fan 

 rays toward the free margin of the fin. So closely are these Clements 

 then fused that it becomes impossible to determine their juncture; 

 the histogenesis of the dermal rays appears to offer the same 

 characters as demonstrated in Teleosts by Harrison ^). Another 

 interesting feature in the larval development of Ämia is the mode 

 of appearance of the jugular plate. This, although an unpaired 

 structure, might reasonably on purely a priori grounds be expected 

 to have had a paired origin, — that is if we accept the view that 

 the jugular plates in the various sub-groups of the early Teleostomes 

 have not been involved independently. It is accordingly of great 

 interest that in this stage the jugular plate, PI. 10, Fig. 19 a jug.^ 



1) in: Arch. Anat, V. 32, p. 248—278. 



