GANG I DEI. 



8!) 



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

 The substance of which they are formed corresponds morphologically to 

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

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

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

 larval life. 



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

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

 in the embryo of Lepidosteus (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 ^~^~~^~ Q P 



the barbels, since these oc- 

 cupy a precisely similar po- 

 sition 1 . 



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. 54, ol\ JY"' 54> Sll)E VIEW OF A LAKVA OF ACIPENSEK 



- - - - - OF 11 MILLIMETUES. 



-.st. 



pp. 



m. 



<>/i. eye; ol. olfactory pit; st. suctorial (?) pro- 



It is probable that this flap 



is equivalent to the fold of cesses';', mouth; s 2 ). spiracle; ;/. gills. 

 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 uares of 

 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 FlG " 55. VENTED VIEW OF A LAEVA OF Ac:- 



J ' to _ PENSEE OF 11 MILLIMETRES. 



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



rise to all the unpaired fins 



of the adult, except the first ( j. gills. 



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 alimentary tract. In 



1 It' these identifications are correct the barbels of fishes must be phylogenetically 

 derived from the papilla; of a suctorial disc adjoining the mouth. 



