196 ME. W. K. PAEKEB ON THE STEUCTUEE AND DEVELOPMENT 



To elucidate the meaning of the oral visceral arches more thoroughly, I have given a 

 figure (PI. XXXIV. fig. 6) of them in their earliest stage, separated from the rest, but 

 shovFn 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 tiie Skate (PI. XXXIX. 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 {au). 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 (PI. XXXV. fig. 5). The embryo which was thus prepared mea- 

 sured Ij^ inch in length — one sixth of an inch longer than the last. 



The mesocephalic flexure was still perfect, the mid brain (6'^) being in front, and the 

 fore brain (C^) below ; the highest part of this object is at the fore part of the notochord 

 (jic), where it is embraced by the hind part of the trabecule. 



Suteh a preparation, then, must be supposed to " dip " both in front and behind, from 

 the postpituitary region; thus the trabeculse [tr) and the investing mass {iv) meet at 

 a consideiable angle. 



The skeletal parts here displayed are much less solid than the visceral rods. This is 

 also the case in Siredoii at a similar stage ; but as the granular trabeculae took up the 

 carmine much more freely than the rest, and could be mistaken for no other tissue 

 than young hyaline cartilage, I have coloured it as such. 



But the cells forming the investing mass [i v) and the shell of the ear-labyrinth were 

 much less coherent than those of the trabeculse ; 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 a? 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. 



