194 PROF. T. H. HUXLEY ON MENOBRANCHUS. [Mar. 17, 



converge and coalesce into the internasal prolongations, which give 

 rise to the mesethmoid of the adult Frog. But, in the tadpole, at 

 this stage of its development, the "parachordal cartilages," which 

 have been developed at the sides of the notochord, have united with 

 one another and with the trabeculse, and thus the pituitary space is 

 much shorter than in Menobranchus. The cartilaginous skull of a 

 tadpole of this age, in fact, has already obtained a higher develop- 

 ment than it ever reaches in Menobranchus. 



The auditory capsules are rounded behind, in the tadpole, and do 

 not extend backwards as pointed processes beyond the level of the 

 exoccipitals ; in which respect the tadpole's skull is more frog-like, 

 and less fish-like, than that of the adult Menobranchus. 



In the tadpole's skull, the suspensorium is attached to the trabecula 

 of its side, close to the point at which the latter passes into the para- 

 chordal cartilage. The cartilaginous band (m, Plate XXXI. fig. 3), in 

 fact, which passes into the trabecula, is the dorsal end of the mandi- 

 bular arch, and corresponds with the pedicle of the suspensorium in 

 Menobranchus, having the same relations to the ganglia and branches 

 of the fifth and seventh nerves. In the adult Frog, the pedicle of the 

 suspensorium has been carried outwards by the lateral growth of the 

 auditory region of the skull, and is articulated by a joint* with the 

 cartilage of this region, close to the outer extremity of the transverse 

 arm of the parasphenoid. The inner process of the pterygoid lies on 

 its ventral side, closely applied to it. 



The elbow (o) by which the suspensorium of the tadpole abuts 

 against the anterior and external face of the auditory capsule 

 evidently corresponds with the otic process of the suspensorium of 

 Menobranchus. In the adult Frog, the suspensorium, which is 

 ossified only at its mandibular end, forks, at its cranial end, into two 

 branches or crura, the interspace between which is filled by fibrous 

 tissue (Plate XXXI. fig. 6). These crura, and the fibrous tissue 

 which connects them, form the front wall of the tympanic cavity : 

 the dorsal crus, which answers to the otic process, passes into the 

 tegmen tympani, or roof of the tympanum, which is furnished by 

 the outgrowth of the auditory capsule ; the ventral crus is the pedicle 

 of the suspensorium just mentioned. 



Passing between the two crura (as Duges long since pointed out) 

 the seventh nerve enters the tympanum, closely applied to the inner 

 wall of which (but not included in any Fallopian canal) it passes, 

 above the level of the fenestra oralis, over the columella auris. It 

 takes, in fact, exactly the same course as in a mammal, except that 

 it runs round the auditory capsule, instead of being included in a 

 canal by the growth of the latter round it. 



Some remarkable consequences appear to flow from the observed 

 metamorphoses of the cranial end of the mandibular arch in the 

 Frog. If the ossification which has already set in in the mandibular 



* My friend Mr. Parker, F.K.S., in his remarkable memoir on '-The Struc- 

 ture and Development of the Skull of the Common Frog " (Philosophical 

 Transactions. 1871), has given a different account of the origin of this singular 

 articulation ; but I believe I may say that he now agrees with me. 



