754 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



most marked in the region of the trunk (Plate 36, fig. 37). The 

 sub-notochordal rod (s. nc.} lies close under it. 



A completely closed fore-gut, lined by thickened hypoblast, 

 extends about as far back as the auditory sacks (Plate 36, figs. 35 

 and 36, a!.). In the trunk the hypoblast, which will form the 

 walls of the alimentary tract, is separated from the notochord 

 by a considerable interval. 



Ninth day after impregnation: External characters. Very 

 considerable changes have taken place in the external characters 

 of the embryo. It is about 8 millims. in length, and has assumed 

 a completely piscine form. The tail especially has grown in 

 length, and is greatly flattened from side to side : it is wholly 

 detached from the yolk, and bends round towards the head, 

 usually with its left side in contact with the yolk. It is pro- 

 vided with well-developed dorsal and ventral fin-folds, which 

 meet each other round the end of the tail, the tail fin so formed 

 being nearly symmetrical. The head is not nearly so much 

 folded off from the yolk as the tail. At its front end is placed 

 a disc with numerous papillae, of which we shall say more here- 

 after. This disc is somewhat bifid, and is marked in the centre 

 by a deep depression. 



Dorsal to it, on the top of the head, are two widely separated 

 nasal pits. On the surface of the yolk, in front of the head, is to 

 be seen the heart, just as in Sturgeon embryos. Immediately 

 below the suctorial disc is a slit-like space, forming the mouth. 

 It is bounded below by the two mandibular arches, which meet 

 ventrally in the median line. A shallow but well-marked de- 

 pression on each side of the head indicates the posterior boundary 

 of the mandibular arch. Behind this is placed the very con- 

 spicuous hyoid arch with its rudimentary opercular flap ; and in 

 the depression, partly covered over by the latter, may be seen a 

 ridge, the external indication of the first branchial arch. 



Eleventh day after impregnation : External characters. The 

 embryo (Plate 34, fig. 10) is now about 10 millims. in length, and 

 in several features exhibits an advance upon the embryo of the 

 previous stage. 



The tail fin is now obviously not quite symmetrical, and 

 the dorsal fin-fold is continued for nearly the whole length of the 

 trunk. The suctorial disc (Plate 34, fig. 1 1, s.d.} is much more 



