DEVELOPMENT OP THE SKULL IN THE BATRACHIA. 19 



Skulls of larval " Ranid^." 



Chondi'ocraiilum of perfect larva of Bull Frogs. 



First Star/e. — 1. Tadpole of Rana clamata (A), ivhose total lemjUi, was 3\ inches; of 

 the tail, 2 inches ; and of the hind legs, 5 lines (-^Aj *»('/')• 



The skull (Plate 2, figs. 5, 6) has now attained its full cartilaginous {Petromyzine) 

 condition, and has gone beyond it ; there is a pair of cartilage-bones, and there are 

 three membrane bones. I sliall deal with these signs of metamorphic progress after 

 describing the chondrocraniuin. 



The cranium, proper, is now a small flat-bottomed hocit, with a semi-cartilaginous 

 deck ; this has been constructed upon the original frame in the following manner. 



The large open space in the orbital region has acquired a thin floor of cartilage, and 

 this median (" intertrabecular ") tract has grown forwards to the region of the inner 

 nostrils {i.n.), where the trabeculae have grown together, and are diverging again as 

 free, flat, broad-ended " cornua" (c.tr.). 



Behind, the occipital arch is as perfect as may be, for the auditory capsules (vh.) 

 fit into large fenestr£B in its sides ; its top (s.o.) is flat, and extends to the fore- 

 most third of the capsules ; its floor is flat, and contains the diminishing notochord (nc.) 

 in its middle : in this kind this rod scarcely acquires any true cartilage in its outer 

 sheath. 



In the large orbital region the side walls are cartilaginous, except where the optic 

 nerves (II.) pass out of their " fenestrse " — spaces three or four times as large as the 

 actual foramina. 



As in the occipital region, so also in this, there is a cartilaginous "tegmen;" its 

 halves have not united in the middle, and a bilobate " fontanelle " (/o'.) exists between 

 these growths and the superoccipital cartilage. Then there is a large oval fontanelle (/o.) 

 or membranous space up to a short distance of the end of the cranial cavity, wh^i-e a 

 tract of cartilaginous roof margins this egg-shaped space. 



Here there is a perfect cartilaginous cincture, which becomes in the adult the " os 

 en ceinture ;" its front end is extended as a cartilaginous beak formed from below 

 upwards as a crested tract of the " intertrabecula ;" this is the mesethmoid (p.e.) in 

 rudiment ; the lateral regions are the right and left " ecto-ethmoids^' (ale.). 



These lateral parts grow outward into wings whose scooped front margins form a 

 back wall to the nasal sacs ; the olfactory nerves pass to these sacs through " fenestras " 

 that lie between the middle and outer parts of the cincture. 



The nasal roofs are still soft, and in the dissection the olfactory sacs were removed 

 to display the cartilages. 



But the internal nostrils (i.n.) are shown in these figures ; they are large and 

 neatly circular ; much of their margin is seen to be formed by the narrow, clinging 

 part of the trabeculie, and the curved spattdce that grow out from the ethmoidal 



