DEVELOPMENT OF THE SKULL IN THE liATllAUHlxV. 33 



Fifth Stage. — 1 (continued). Advanced Tadpoles o/'Eana clamata (C), all limbs free; 



3^ inches long ; tail, iW inch long. 



If this (Plate 4, figs. 5-7) be conipared with the Third Stage, the second larva of 7?. 

 clamata, (figs. 1, 2) we shall see what approach is made towards the adiUt condition. 

 Tlie angular and almost oblong form of the whole structure is changed for a short oval ; 

 and the facial outworks are only half the real and relative width they possessed then. 

 The auditory capsules are of greater extent, antero-posteriorly, the ex-occii^itals (e.o.) 

 are larger, and the prootics {pr-o.) have begun on the outside of t\\e foramen ovale (V.). 

 The form of the cranial cavity is narrower in front compared w'ith the hind part, and 

 the whole ethmo-nasal structure in front of the cavity is shorter, and much modified. 



Besides the septum nasi {s.n.) the whole intertrabecular space is filled m in front ; 

 the " cornua " {c.tr.) are only half as long, and are bifurcated, the outer fork has become 

 the large arcuate angle of the "subnasal lamina," and the lesser (inner) fork is the 

 hooked " pro-rliinal " {p.rh.) which lies inside the premaxillary {px.), and curves 

 backwards. 



The mternal nares {i.n.) are now relatively much forwarder, are more elongated, 

 and are placed quite transversely. 



The front wall of the nose is completed by fusion of the roof wdth the septum, and 

 a growth of cartilage over it to the lower face of the labyrinth. The external nostrils 

 {e.n) are now also much nearer the front of the head, for the large labial plate has 

 been absorbed and two pairs of small new plates that form the outworks of the nose 

 have appeared. The inner pair {u.l^.) are applied to the mner face of the apex of each 

 nasal pi'ocess of the premaxillaries (pa;.) ; whilst the outer forms a shell-like valve 

 to the external margin of the nostril {u.l~., e.n.). 



The nasal bones (a film over na.) have appeared like small shells. The condyle of the 

 quadrate (q.) is now opposite the hinder margin of the nasal roof, instead of the front 

 margin ; it is twice as far from the roof, and its condyle looks obliquely outwards, and 

 not inwards as before. The elongated and obhque palato-pteiygoid bar- now has the 

 " post-palatine rudiment " as a ridge looking outwards and backwards, and the " pre- 

 palatine spur " (pr.jpa.) nearly touches the tip of the horn of the trabecida (angle of 

 "sub-nasal lamina," c.tr.). 



Tracing the narrowing and retreating suspensorium backwards, we see that the 

 orbitar process, and condyle for the hyoid [or.jx, hyf.) are now opposite, not the 

 antorbital region, but the middle of the subocular fenestra. 



Further back, the true "otic process" is small and rounded (fig. 5, ot.p.), and the 

 spiracular cartilage, now detached from the tegmen tympani, apphes itself to the side 

 of the otic process {sp.c, ot.p., t.ty.) ; it is now a trifoliate rudiment of the "annulus." 

 Below (fig. 6, pd), the pedicle is forming its condyle, to glide upon a pre-auditory 

 process of the basal plate, whilst the dorsal part of the pier is now an attenuated 

 thread of cartiLige running under the prootic to the front of the foramen ovale (Y.). 



MDCCCLXXXI. F 



