10 ILLINOIS BIOLOGICAL MONOGRAPHS [10 



Another cartilage arises from the lateral side of each trabecula, behind 

 the choana, and the anterior margin of the cristal connection of the trabec- 

 cula and column. This has been called by the German writers, the palatine 

 process; but it has no relation to the palate or the palatine bone, and is 

 better called by another name, frequently used for it, the processus antor- 

 bitalis (pa). It extends laterally about as far as the cornu, and is destined 

 to form a part of the posterior wall of the nasal capsule. 



As yet there are no distinct olfactory foramina, but the olfactory nerve 

 leaves the forebrain at right angles to its median axis and passes into the 

 capsule through the large gap between the trabecula and the column, just 

 anterior to the crista trabeculae. 



In a 34 mm. larva (Figs. 5, 6) the process of chondrification which formed 

 the pons ethmoidalis of the 25 mm. stage, has continued ventrally and 

 posteriorly, so that the bridge has joined the planum basale and has extend- 

 ed back to, about the level of the planum tectale of the previous stage. 

 The result of this is the formation of a large median wall to the cavum 

 cranii, the only openings left being the olfactory foramina which pass into 

 the capsules at the lateral margins of this wall. From conditions which 

 occur in other Urodeles this median mass is best called the planum Verticale, 

 although it is much thicker than in Amphiuma and the Caecilians. As will 

 be seen in the sequel, this planum verticale may be denned as the cartilage 

 connecting the nasal capsules of the two sides, beginning as a median dorsal 

 pons ethmoidalis from the two columnae and then extending down to join 

 the planum basale. The anterior margin of the planum verticale (pv) is 

 deeply excavate, and together with the medial walls of the nasal capsule, it 

 bounds the V-shaped internasal space in which the intermaxillary glands 

 lie. The lateral margins of the verticale slightly extend over the medial 

 margins of the nasal organs from the olfactory foramen to the tip of the 

 capsule, thus forming the anterior part of the planum tectale, which is 

 pierced by a small opening (fni ) through which the ramus nasalis internus 

 of the fifth nerve passes into the internasal space. Thus the planum 

 verticale is a compound structure formed from the ethmoidal bridge and 

 the columnae ethmoidales, uniting ventrally with the planum basale. 



The posterior parts of the planum tectale (pt), which began in the last 

 stage (25 mm.), now forms a broad curved plate which covers the caudal 

 half of the nasal organ, and extends forward from the crista trabeculae to the 

 level of the anterior margin of the foramen olfactorius. Laterally it extends 

 nearly to the level of the gap between the caudal extension of the cornu and 

 the antorbital process; the posterior margin of the tectale is oblique, its 

 antero-lateral margin is slightly arcuate, while in front it is produced into a 

 small conical process (Figs. 5, 6) . 



The expanded cornua trabeculorum (ct) do not differ greatly from those 

 of the preceding stage. The lateral margin of each cornu is slightly arcuate 



