192 ME. W. K. PAEKER ON THE STEUCTURE AND DEVELOPMENT 



skull in the Common Frog', will show what is seen in Biifo vulgaris, jDactylethra 

 capensis, Pij)a monstrosa, Siredon (with Amhlystoma), Seironota, and the adult skull of 

 the lowest Amphibian, namely that of Proteus anguinus. 



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 

 " trabeculse 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 trabeculse 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 trabeculse belonged to the 

 visceral series — its terminal arch; then I yielded to Professor Huxley's view of the 

 visceral nature of the rods throughout ; noiv 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 Spotted Dog-fish 



(Scylliimi canicula). 



First Stage: Embryo of Bog-Jish, 8 lines toll 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 (PI. XXXIV. fig. 1) is intermediate 

 between the first and second stages described in the Frog (" Frog's Skull," pis. 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 (C) loking downwards. Everywhere very translucent, 



' "Batrachia," Part 2, is now in print. See Phil. Trans. 1876, part 2, pp. 601-669, plates 54-62. 



