DEVELOPMENT OE THE SKULL IN STURGEONS. 
151 
two-fifths of the whole length, the long arched notochord is ascending towards the upper 
part of the continuous azygous crest, and three-fifths of the caudal part of the vertical 
fin, above and below, is being marked off, ready to form the proper heterocercal tail- 
fin; the narrow crest above develops the “ fulcra,” and the wider part below the emar- 
ginate lobe. The high crest in front of the constriction becomes the backwardly- 
placed dorsal fin, and the deep crest, below, the anal fin ; the ventral fins (v.f.) have a 
rudiment on each side as a thickening of the mesoblast of the abdominal wall close in 
front of the vent (an.) ; the pectoral fins (p.f.) are large and reniform. The region of 
the stomach still contains enough yolk to give it a swollen appearance ; the intestinal 
region (in.) is narrow. The regions of the brain are still clearly seen from the 
outside ; the eye ball (e.) is nearly perfect; the auditory capsules (an.) are seen 
as relatively small oval swellings. 
The nasal capsules (ol.) have their opening almost subdivided into two by the 
descending flap. The suborbital fold of the face over the barbels is now large, 
and they (bb.) are long and finger-like. The lips (u.l., LI.) are now thick and rounded; 
the upper is emarginate in the middle, and the lower has a medium lobe ; the teeth 
(t.) are now inside the lips. The spiracle is now a very small triangular hole 
under the auditory capsule (Plate 12, fig. 9, cl 1 .) ; the operculum (op.) although very 
large and extensive does not cover the copious growth of young gills. 
Dissections of the cranio-facial skeleton of a specimen 13f millims. long show, already, 
a well chondrified framework. The basis cranii, as seen from above (Plate 14, fig. 4), 
shows a structure very similar to that of an embryo of Scyllium canicula 1-J inch long 
(see Zool. Trans., vol. x., plate 35, fig. 6), or more than twice as long as this embryo 
Sterlet. 
The cartilage investing the notochord (Plate 14, fig. 4, iv., nc.) is continued from the 
spinal into the cranial region with scarcely any change of form and consistence, a state 
of things quite like what I have found in the Selachians (op. cit. , plate 35, figs. 3, 5). 
In my preparation four pairs of spinal nerves were seen emerging behind the glosso¬ 
pharyngeal and vagus (Plate 14, fig. 4, sp.n 1 ' 1 ., X., IX.). The bands of cartilage, 
whose size and form will be further shown in the sections (Plates 14 and 15) are con¬ 
fluent above for a short distance with the auditory capsules (iv., au.) close behind the 
post-auditory nerves (IX., X.). In front of these conjugations there is a large open 
space, the “meatus internus” (VIII.), between the cartilage of the capsule and that of 
the skull floor; the latter widens a little in front of the meatus, then narrows a little to 
form the inner margin of the foramen ovale; outside this notch we see the huge 
Gasserian ganglion (V.). The investing mass then widens a little, and ascends a little ; 
this wider ascending part is the “posterior clinoid” wall (p.cl), and in it the notochord 
ends as a small rounded knob. 
As in Scyllium (op. cit., plate 35, fig. 6) the bands of cartilage then dip consider¬ 
ably, and these prochordal continuations (tv.) of the parachordal tracts, are wider than 
the roots they spring from. Instead of being wide apart, as in the Salmon, in Lepi- 
