18 MR. W. K. PARKER ON THE STRUCTURE AND 



Behind these parts, in relation to the hinder visceral clefts, four other ai'ches are 

 beginning to form. These, the branchials, are still more feebly developed, never acquire 

 an upper element (" epi-branchial "), but in their arrest and feebleness are supplemented 

 by four j^airs of extra-hranchial cartilages or pouches ; these I shall describe in the 

 developed embryos of the larger species of the genus liana. 



Until then, also, I shall not describe the other superficial cartilages that are found in 

 the Tadpole's head, viz : that over the " 1st cleft," and those so largely forming the 

 suctorial mouth of the Tadpole. 



In this early stage the cranial nerves pass over the trabecvdse through the memhrano- 

 cranium. 



Here, if anywhere, we ought to find rudiments of pre-oral arches, if there be any ; 

 there are certain tilings in other kinds of embryos, and even adult forms, that suggest 

 such a possibility, but none here."' 



At present the mandibular pier has two points of fusion with the trabecula ; the 

 hinder of them is the pedicle {pd.), and the front one the palato-pterygoid bar {p.pfj.) ', 

 the pedicle will be largely absorbed, and the palato-pterygoid enormously developed 

 during the metamorphosis of the Tadpole. 



Before passing to the description of the well-developed Tadpole, I must refer again 

 to the intermediate stages (ibid., Plate 55, figs. 3, 4, 5). 



On its way to the coronoid region of the mandible, the temporal muscle passes over 

 the palato-pterygoid bar, and under a leafy growth of the "suspensorium" — the "orbitar 

 process " {or. p.). In Bufo vulgans the apex of this leaf coalesces with the ethmoidal 

 cartilage, a rare character. This fact, and its nonconformity with what is seen in the 

 Tadpole of the Frog, wei'e pointed out to me by Professor Huxley five or six years ago. 



In other species (even of Bufo) I do not find this very peculiar condition of things, 

 but the process itself is well developed, even in the wide-mouthed larva? of the 

 " Aglossa " (ibid., Plates 56-61). 



In the half-grown larvse figured in my pajier (ibid., Plate 55, figs. 4, 5, 5a), we see 

 the formation of the " fenestra ovalis " as an obhque lateral cleft in the auditory 

 capsule ; and of the " stapes" as the solidification of the soft tissue left in this space. 

 In larvae three-fourths grown ossification begins in the tissue over and under the 

 chondrocranium. 



* Professor ^VfiLNES Marshall and the writer, from a consideration of the "segmental nerves" of the 

 liead, from the development of the mouth itself and of the lacrymal and nasal passages, and from what is 

 seen in various cartilages around and in front of the mouth, hold to the oplnioti that there are rudiments of 

 at least tioo, perchance titrce, prc-oral visceral arches. 



But Professor Huxlet and Mr. F. M. Balfoir will not see with our eyes, and are not in the least 

 satisfied with the evidence whicli seems so conclusive to us. The Lamprey and the Tadpole are indeed 

 great stumbling-blocks; if they represent primordial or archaic forms, and if what we see in other kinds 

 bear to be interpreted as specialisations having relation to modification of the oral aperture and its 

 framework, the onus probandi will still rest with my talented young friend Marshall and me. 



