196 MR. W. K. PARKER ON THE STRUCTURE AND DEVELOPMENT 
To elucidate the meaning of the oral visceral arches more thoroughly, I have given a 
figure (Pl. XXXIV. fig. 6) of them in their earliest stage, separated from the rest, but 
shown in relation to the three infolded sense-capsules. Looked at unreflectingly, the 
mandibular and hyoid arches might be supposed to have their apex turned completely 
forward, so that the proximal part of the first might be brought to the distal end of the 
trabecular arch or plate; whilst the other, the hyoid, should have its apex close behind 
the foramen ovale (5). A careful observation of these, and comparison of them with those 
of the Skate (Pl. XX XIX. fig. 3) has satisfied me that the fore-turned hook is in 
reality a secondary fork, growing in these two arches from the primary fixing-point. 
Thus we still have the first position of the mandibular apex below the trigeminal nerve, 
and that of the hyoid below the auditory sac (aw). Now, if the “third postoral” or 
first branchial be compared with these two, it will at once be seen that it has, like its 
successors, no fork, and that its apex corresponds with the heel of the foot-shaped top 
of the first and second bars, 
We shall see, anon, what becomes of the pedate process, and of the proper apex from 
which it proceeds; the forked part is a ‘ conjugational spur,” having the same me- 
chanical meaning as the slanting bars of a simple rural gate. 
'This stage is still further illustrated by a section which shows the floor of the rudi- 
mentary skull; it passes horizontally through the nasal sacs; but the eye-balls and ear- 
sacs are untouched (Pl. XXXV. fig. 5). The embryo which was thus prepared mea- 
sured 1}; inch in length—one sixth of an inch longer than the last. 
The mesocephalic flexure was still perfect, the mid brain (C?) being in front, and the 
fore brain (C1) below; the highest part of this object is at the fore part of the notochord 
(nc), where it is embraced by the hind part of the trabecule. 
Such a preparation, then, must be supposed to “dip” both in front and behind, from 
the postpituitary region; thus the trabeculz (¢7) and the investing mass (7 v) meet at 
a considerable angle. 
The skeletal parts here displayed are much less solid than the visceral rods. This is 
also the case in Siredon at a similar stage; but as the granular trabecule took up the 
carmine much more freely than the rest, and could be mistaken for no other tissue 
than young hyaline cartilage, I haye coloured it as such. 
But the cells forming the investing mass (¢v) and the shell of the ear-labyrinth were 
much less coherent than those of the trabecule; and the internasal tract was still more 
behindhand in growth. 
The huge swelling brain-sacs, especially the middle one (C*), project far forwards ; 
and traces of growth from the axis can be found as far forwards as the front of the 
elegantly plaited nasal sacs (na). 
If the subcircinate series of structures in the head of this Selachian embryo be con- 
sidered, it will be seen that there are three pairs of sense-capsules, and their inter- 
capsular regions, the auditory, the optic, and the olfactory interspaces. 
