GANOID EI. 



89 



Fig. 5-i. Side view of a larva of Acipenser 



OF 11 MILLIMETRES. 



op. eye; ol. olfactory pit ; st. suctorial (?) pro- 

 cesses; VI. mouth; sp. spiracle; g. gills. 



deposit between a papilla of subepidermic tissue and an epidermic cap. 

 The substance of which they are formed corresponds morphologically to 

 the enamel of ordinaiy teeth. As they grow they pierce the epidermis, 

 and form hollow spine-like structures with a central axis filled with sub- 

 epidermic (mesoblastic) cells. They disappear after the third month of 

 larval life. 



In front of the mouth two pairs of papillse grow out, which ap- 

 pear to be of the same nature as the papillas on the suctorial disc 

 in the embryo of Lepidosteas {vide p. 95). They are very short in 

 the embryo represented in fig. 53; soon however they grow in length 

 (figs. 54 and 55, st) ; and it 

 is probable that they become 

 the barbels, since these oc- 

 cupy a precisely similar po- 

 sition \ 



, The openings of the na- 



/ sal pits are at first single ; 



)but the opening of each be- 



\ comes gradually divided into 

 two by the growth of a flap 

 on the outer side (fig. 5-i, ol). 

 It is probable that this flap 

 is equivalent to the fold of 



/the superior maxillary pro- 

 cess of the Amniota, which by its growth roofs over the open 

 groove which originally leads from the external to the internal nares ; 

 so that the two openings of 

 each nasal sack, so estab- 

 lished in these and in other 

 fishes, correspond to the ex- 

 ternal and internal nares of 



j higher Vertebrata. 



At the time of hatching 

 there is a continuous dorso- 

 ventral fin, which, by atrophy 

 in some parts, and hyper- 

 trophy in other parts, gives 

 rise to all the unpaired fins 

 of the adult, except the first 

 dorsal and the abdominal. 

 The caudal part of the fin is at first symmetrical, and the hetero- 

 cercal tail is produced by the special growth of the ventral part of 

 the fin. 



Of the internal features of development in the Sturgeon the most 

 important concern the relation of the yolk to the aliraentaxy tract. In 



' If these identifications are correct the barbels of fishes must bo phylogenetically yi 

 derived from the papillae of a suctorial disc adjoining the mouth. » \ 



— -#■-!'— **J 



Fig. 55. Ventral view of a larva of Aci- 

 penser OF 11 MILLIMETRES. 



m. mouth ; st. suctorial (?) processes ; op. eye ; 

 g. giUs. 



/ 



