192 MR. W. K. PARKER ON THE STRUCTURE AND DEVELOPMENT 
skull in the Common Frog’, will show what is seen in Bufo vulgaris, Dactylethra 
capensis, Pipa monstrosa, Siredon (with Amblystoma), Seironota, and the adult skull of 
the lowest Amphibian, namely that of Proteus anquinus. 
The reader, if he compare this with my former papers on the same subject, will find 
that my views as to the morphological interpretation of the parts are not fixed, but 
oscillate, tentatively. 
I hold that this vacillation is safest at present; in each succeeding paper I express 
the views that seem to me to be true at the time; and I would rather waver in doubt— 
working upwards towards the light—than become fixed in the belief of some favourite 
view that might turn out to be a mere verisimilitude, and essentially erroneous. 
No man at present is able to say whether all or part, or, if part, how much of the 
“ trabecule cranii” are ventral, or belong to the visceral-arch series. 
It was not known until lately whether there were any true visceral arches between 
the great mandibular arch and the “horns” of the trabecule in the frontal wall of 
the face. 
The interpretation of the cranial nerves is extremely difficult when it is sought to 
arrange them as the serial homologues of the spinal nerves—and this not merely in the 
nerves of special sense, but also in the common motor and sensory nerves, such as the 
“trigeminal,” “ facial,” &c. 
My own opinion was that the facial part only of the trabecule belonged to the 
visceral series—its terminal arch; then I yielded to Professor Huxley’s view of the 
visceral nature of the rods throughout; now I sway back again, and think that their 
subcranial part is axial in nature. 
Also as to the relation of the “visceral” to the “costal” arches, here is another 
heavy difficulty: in morphological “ habit” they are diverse exceedingly ; and whilst the 
latter are developed in a continuous “‘somatopleure,” the former are solidifications of 
the cloven oro-faucial wall. 
ON THE DEVELOPMENT OF THE SKULL AND FACE IN THE LessER Sporrep DoG-FrisH 
(Seyllium canicula). 
First Stage: Embryo of Dog-fish, 8 lines to 11 lines in length. 
Keeping the development of the Frog’s skull in view, it may be remarked that, on 
the whole, the youngest embryo of the Dog-fish (Pl. XXXIV. fig. 1) is intermediate 
between the first and second stages described in the Frog (*‘ Frog’s Skull,” pls. 3 & 4) ; 
and with these they may be compared. 
The “ mesocephalic flexure” was complete, the middle vesicle (C*) projecting for- 
wards, and the anterior vesicle (C1) loking downwards. Everywhere very translucent, 
* “Batrachia,” Part 2, is now in print. See Phil. Trans. 1876, part 2, pp. 601-669, plates 54-62. 
