16 WR. W. K. PARKER ON THE STRUCTURE AND 



I must here remark that the early cranium of the Aglossal types, especially that of 

 Dactylethra, is in some respects more Petromyzine than that of an ordinary Tadpole. 



In some things the chondrocranium of the Tadpole of Bu/o vulgaris comes nearest to 

 that of the Lamprey, as in the fusion together of the various plates of cartilage ; 

 but the skiUl of the larval Daftijlethra comes very near to that of the adult Petromijzon 

 in several respects. 



This is seen in the complete fusion of the elements of the branchial skeleton, and in 

 the histological condition of those bai's and pouches : these are composed, as in the 

 extra-branchial basket-work of the Lamprey, of a very light kind of cartilage, with 

 large cells and scarcely any intercellular substance : the Inrge amount of superficial 

 cartilage shows also the same relationship. 



The Common Toad differs from most of the Opisthoglossa in retaining the primordial 

 "pedicle" to the pier of the mandible, which is, from the first, continuous with the 

 trabecular bar : this is a remarkable sui\ival of a suctorial character — even after 

 metaniorpliosis. 



SKULL OF LARVAL BATRACHIANS. 



First Stage. — Craniofacial cartilages of emhryo of Bufo vulgaris soon after hatching ; 



\rd of an inch in total length. 



Nearly all the truly cartilaginous part of this cranium ("Skull of Batrachia," Plate 55, 

 figs. 1, 2) lies at the base of the fore half of the head (tr.); two globes of soft cartilage 

 imfinished in their upper third, are to be seen right and left in the liind half : these 

 are the very distinct auditory capsules (au.). 



At present the whole body of the embryo exists as a sort of tail-like appendage to 

 the huge and precocious head : the development of cartilage appears, therefore, almost 

 entirely in front of the axis of the organism (notochord), which stops at the post- 

 pituitary space, before the " pituitaiy body" is formed. 



Thus the cranium, as yet, is nearly all p'O-chordal ; and the jjaro-chordal part, like 

 the side-walls and roof of the cranium, is still membranous. 



Yet these prochordal tracts, or " trabeculse cranii " are manifestly true paraxial 

 elements or parts ; they are homologous with the paired cartilages that appear in any 

 region along the sides of the sheath of the notochord. 



All growths above these (dorsad) are of the nature of neural arches ; all growths 

 beloiv these (ventrad) are " visceral" or "pleural" arches. 



But these rods ai-e continuous, and they are not parallel the one with the other ; 

 how is this to be explained on the theory of their j^ctrcixial nature ? 



A consideration of the development of this, or of any. Vertebrate embryo, will help 

 us to understand the meaning of these first foundations of the cranium. 



The neural axis rapidly enlarges at its cephalic end, forms three vesicles there, the 

 hind, mid, and fore brain, and this beaded structure is bent suddenly upon itself so 



