DEVELOPMENT OE THE SKULL IN THE BATEACHIA. 
621 
From this point to the posterior edge of the pterygo-palatine bar is the nasal region, 
and, with the help of the upper labials, this tract grows up into all that labyrinth. 
The trabecular cornua in front of that process grow, indeed, as long as the larva is a 
larva, and afterwards give off the “ prorhinals ” (Plate 54. figs. 3, 4, c.tr.). 
The prenarial ligament in the Tadpole binds together the rudimentary alinasal floor 
and the inner face of the quadrate (q.) ; the corresponding point of the quadrate in the 
adult is exactly opposite the anterior margin of the exoccipita] bone, which corresponds 
with a line drawn across through the middle of the cranial notochord *. 
The auditory sacs (Plate 55. fig. 3, au.) are now obliquely pyriform ; the cartilaginous 
coat, stretched almost to bursting by the rapid growth of the labyrinth, has yielded to 
the form of that which it bears within. An elegant, large, oval fenestra still exists on 
the top of the capsule, towards its inner side. This is the counterpart of the membra- 
nous space which lingers for some time in the roof of the capsule in the embryo of the 
Salmon (“ Salmon’s Skull,” plates i., ii., iii., au.) ; this is the remains of the “ aqueduct ” 
or primary involution of the ear-sac (see “ Pig’s Skull,” p. 299, plates xxviii. & xxix.). 
The huge semicircular canals shine through the hyaline cartilage and their fenestral 
membrane (fig. 3). 
Skull of Bufo vulgaris. — Fourth Stage. Tadpoles 8 lines long. 
In Tadpoles twice the length of the first stage here given, viz. two thirds of an inch 
long, I obtain a view of the substance, now considerably differentiated, which becomes 
the stapes. Professor Huxley had suggested that my first view (“ Frog’s Skull,” p. 157) 
was incorrect, and that it was not segmented out from the already cartilaginous capsule, 
but formed later, by chondrification of the tissue lying in the fossa or primitive “ fenestra 
ovalis ” (see fig. 4 ,f.s.o.). I had long before found a cleft in the floor of the capsule in 
ripe embryos of Salamandra maculosa , and it seemed to me that this cleft went on to 
cut old, as it were, a large “ bung.” This process does take place in the “ Urodela ” 
( Siredon , Salamandra), but not in the “ Anura.” The pitting of the sac, below, in Toad 
Tadpoles 5 lines long (fig. 4, au.), is the commencement of a peculiar involution and 
subsequent rupture of the cartilaginous wall (fig. 5, au., st.). By the huge, growing 
anterior and horizontal semicircular canals, and the development of the “ tegmen 
tympani,” the little pit is brought more beneath the capsule; it has also grown 
backwards, as a cleft, absolutely as well as relatively. The involution of the periotic 
wall appears as though it had been made by pushing in with a finger, obliquely, and 
the size of the opening is much less than it appears from the surface. It is an obliquely 
pyriform space, looking inwards, and extending its narrow end backwards ; as is the 
* Eor the farther development of the nasal labyrinth in the Batrachian, I must refer the reader to my 
former paper on the Frog, remarking that the part in the newly metamorphosed Frog (plate vii. fig. 11) does 
not show either the “ labials ” or the prorhinals. In the “ Aglossa ” I shall presently show very simple 
conditions of the labyrinth ; hut the most perfect elucidation will he given when I come to the Paradoxical 
and Bull-frogs. 
