43] THE NASAL ORGAN IN AMPHIBIA HIGGINS 43 



other. In Amblystoma, this process is changed slightly, due to the 

 independence of the ethmoidal columns from the cornua. In the 25 mm. 

 larva, by medial growths of the columnae a pons has formed, so that in 

 this stage the capsule is divided into dorsal and ventral halves, the only 

 connection being the crista. By further chondrification of the region 

 between the pons and the planum basale, a verticale is formed, the process 

 continuing posteriorly, developing the solid thick cartilage of the 45 mm. 

 larva. 



A planum verticale does not chondrify in Triton and Diemictylus, so 

 that internasal space and cavity of the forebrain are separated by mem- 

 branous structures only. The pons ethmoidalis develops in Triton, uniting 

 the dorsal medial margins of the capsules somewhat anterior to the planum 

 basale, but it is completely lost in Diemictylus. Evidence is lacking as to 

 the origin of the pons in Triton, but Born (1876) describes the process as a 

 continuous growth of cartilage from the trabeculae, and not as independent 

 parts ; it would appear, then, that the pons has probably arisen by growths 

 from the medial surfaces of the capsules, similar to that of Salamandra. 

 In the absence of a pons, Diemictylus appears to be reduced from some 

 form like Triton ; the nasal capsule of these two being very similar in other 

 details. 



The antorbital process is present in all Urodeles, arising in all, except 

 Cryptobranchus, from the ventral margin of the trabecula just back of the 

 choana. It is directed laterally and anteriorly and in most Urodeles unites 

 to the lamina externa, or to the cornu trabeculae, outlining the foramen 

 orbit o-nasalis, through which the nerves of the nasal region enter the 

 capsule. In Necturus and Amphiuma, on the other hand, this process 

 never unites to the more anterior parts of the capsule, so that a foramen 

 is never formed, nor is there any posterior wall to the capsule as in those 

 Urodeles where the antorbital unites to the tectale, as in Plethodon and 

 Amblystoma. 



Gaupp (1893) held that the antorbital process of Urodeles is homologous 

 with the pterygo-quadrate arch of the Anura; and the terms palatine 

 cartilage or ethmo-palatine have been indiscriminately used when referring 

 to this structure. If we regard Cryptobranchus as ancestral, or at least 

 more primitive, a conclusion I believe both the nasal capsule and the 

 olfactory organs justify, then the relations and the development of the 

 antorbital process in this animal may throw some light upon the homology 

 of these structures. In the two weeks larva, in which the trabecular 

 crests are already well-developed, there is no evidence of an antorbital 

 process; but slightly posterior to its probable position, procartilage cells 

 lie near the lateral ventral margin of the trabecula, which continue pos- 

 teriorly into the pterygoid process of the quadrate. In the later stage, 

 these cells have chondrified and, uniting to the pterygoid, have reached 



