July 30, 1 8 74 J 



NA TURE 



249 



REPORT OF PROF. PARKER'S HUNTERIAN 

 LECTURES "ON THE STRUCTURE AND 

 DEVELOPMENT OF THE VERTEBRATE 

 SKULL"* 



VI. 



T N no animal has the study of cranial development 

 ■*- yielded richer results than in the frog. In tadpoles, 

 from the time of hatching onwards, such points as the true 

 nature of the trabecule, and their distinctness from the 

 investing mass, the fact that the stapes is a segmented 

 portion of the ear-capsule, and not the apex of the hyoid 

 arch, and the relations of the pterygo-palatine arcade 

 have been demonstrated with certainty. Most instruc- 

 tive, also, is the way in which the various arches become 

 segmented, altered in shape, direction, and relative size, 

 and made to subserve the most various functions. 



JirXliy^ 



Fig. 17. — Head of T.\'Jpole 2—3 lines long, with the facial arches exposed 

 by removal of the skin from Ihe left side ( x 6i). 



Desides the adult, nine stages of the frog's skull were 

 described. 



I. (Fig. 17). — In tadpoles at about the time of hatching 

 the whole organism is in a very rudimentary condition. 

 The mouth and the gill-slits are closed, the dehiscence of 

 the tissue between the facial arches by which they are 

 formed not having yet taken place. On the first and 

 second branchial arches small papilla:, the rudiments of 

 the external gills, have made their appearance (see Fig. 

 17). The little creature, now about a quarter of an inch 

 or less in length, is usually found attached to water 

 weeds by the horseshoe-shaped sucker beneath its throat, 

 which, though serving the same purpose, must on no 



Tlyia 



IFic. 18.— Head o( Tadpole, 5 lii 



d,-:p 



long (x 6J). Or.P. Orbitar process. 



account be confounded with the suctorial mouth of 

 the lamprey. The facial arches are in a perfectly 

 simple and undivided condition, all those behind the 

 mouth are curved slightly backwards in the lower half, 

 while the trabecute incline forwards and are thus 

 made to diverge considerably from the mandibulars, 

 although in their upper portion they have almost exactly 

 the same inclination as their successors in the series. The 

 investing mass consists of two small patches of nascent 

 cartilage, one on each side of the notochord. The audi- 

 tory are the only sense-capsules ;which have undergone 

 chondrification, and in them the process is quite incom- 

 plete, a large membranous space being still left uncovered 

 by cartilage. Two pairs of labial cartilages (1) are formed, 

 and probably answer in a general way to the first and 

 fifth of the series described in the shark (see Fig. 2, 

 1-, 1=). 



* Continued fr-m p 1C8. 



2. Tadpole about I in. long). The external gills have 

 now (four or five days after hatching) become plumose 

 and the mouth and branchial clefts open freely into the 

 pharyngeal cavity. The most important advance is in the 

 commencing separation of a small segment (hypo-mandi- 

 bular) from the second arch, which in the next stage has 

 become Meckel's cartilage. The hyoid has also begun to 

 diverge from its predecessor in its lower part, and a fourth 

 branchial arch has appeared in addition to the three ob- 

 servable in the first stage. 



3. (Tadpole about i in. in length, Fig. 18.) The trabe- 

 cule have now united with the investing mass and with 

 each other before and behind the pituitary body, and 

 have become almost horizontal ; they likewise begin to 

 foreshadow some of the changes which afterwards take 

 place in them, becoming slender anteriorly, to form the 

 cornua trabeculs (H.Tr), and just behind the olfactory sac 



Viq.lB. 



Fig. 19, -Head of Tadpole, i in. long ( >'■ 4]). Op. Opercular aperture. 



being thickened slightly in the future ethmoid. il region. 

 Meckel's cartilage now forms a true articulation with the 

 fixed or suspensorial portion of the arch to which it be- 

 longs ; slightly above the articulation two processes are 

 sent out from the suspensorium ; the outer (Or.P) is the 

 oibltar p/vcc-ss, while the inner (Pl.Pt), uniting with the 

 trabecula. forms a commissural band of cartilage, the 

 rudiment of the pterygo-palatine arcade : between these 

 two processes, the second and third divisions of the tri- 

 geminal nerve run. The hyoid arch has assumed a won- 

 derfully shark-like character (see Fig. 2), having divided 

 into an upper and a lower segment, the former of which 

 (hyo-mandibular, H.M) has come into close relation 



g Frog, with tail just absorbed ( ^ ) 5). 



with the preceding arch, while the latter hangs free, 

 forming an open angle with the mandible, and unites with 

 its fellow of the opposite side by means of a median basi- 

 hyal. The investing mass and ear-capsules are now com- 

 pletely cartilaginous. 



4. (Tadpoles i in. long. Fig. 19.) At this stage the 

 hind limbs have made their appearance, and the oper- 

 cular fold has completely grown over the gill arches on the 

 right side, a small slit (Op) still remaining on the left. 

 The cranial elements have now assumed somewhat the 

 appearance of a skull, which however differs most 

 markedly from that of the adult frog. The trabeculse, by 

 complete union in their hinder two-thirds with each other 

 and with the investing mass, have formed a solid basis 

 oanii; they have also sent up a low wall on either side of 

 the brain, thus tending to inclose it, and just in front of 

 their union with the pterygo-palatine have developed a 

 prominent transverse ridge (Fig. 19, Eth),the rudiment of 



