41 ] THE NA SA L ORGA N IN A MPIIIBIA HIGGINS 41 



of the ethmoidal column in Cryptobranchus, for in the five weeks larva it 

 already unites the medial margin of the cornu to the crista; but judging 

 from its relation to the capsule, and in its complete separation from its 

 mate, these structures resemble those of Spelerpes in which the anterior 

 part of the columna arises from the medial margin of the cornu and grows 

 posteriorly along the nasal sac. Further, the anterior prolongation of the 

 dorsal crista in Spelerpes, interpreted as the posterior part of a developing 

 column, may also exist in Cryptobranchus, although conclusive evidence 

 on this point is lacking. 



In the 25 mm. Salamandra larva, the anterior end of the column is 

 connected to the cornu trabeculae much as in Cryptobranchus; and yet 

 the expansion from its posterior end, forming a small tectale prior to any 

 connection with the crista, suggests an independence of the ethmoidal 

 column, like that of Amblystoma. Although the dorsal crests of Salaman- 

 dra develop along with the columna ethmoidalis, the two do not unite until 

 a later stage, thus the posterior part of a developing column is lost in all 

 Urodeles above Spelerpes. In Amblystoma, on the other hand, the eth- 

 moidal column and the cornu trabeculae are chondrified independently of 

 each other. In a 13 mm. larva, I have described a few cartilage cells above 

 the medial margin of the cornu of the left trabecula only. This is the first 

 appearance of an ethmoidal column which subsequently grows posteriorly 

 along the median dorsal margin of the nasal sac. The fact that I observed 

 these cells as chondrifying, at first, upon one side only, has no especial 

 significance; but in the independent chondrification and the complete 

 development of the ethmoidal column from in front backwards, prior to 

 any association with the trabecula, Amblystoma differs from any other 

 Urodele, except Necturus in which the independently chondrified ethmoidal 

 column never unites to other parts of the capsule. Thus in the 20 mm. 

 stage of Amblystoma, completely developed columnae ethmoidales parallel 

 the trabeculae from their anterior tips to the choana; but do not unite to 

 any crista, which here is greatly retarded in its appearance, not developing 

 until the 25 mm. stage is reached. 



The ethmoidal column in Amphiuma is more like that of Cryptobran- 

 chus and Spelerpes in the development of the posterior parts; there being 

 no evidence of an anterior part of a column in the youngest larva studied. 

 Somewhat as in Spelerpes, the anterior portion of the crista continues 

 forward into a cylindrical bar along the medial margin of the nasal sac, and 

 from its relation to other parts, as well as its position, it must be regarded 

 as a columna ethmoidalis. As in Cryptobranchus, it gives off laterally a 

 small process which may be homologized with the tectale, which then turns 

 anteriorly, forming the lateral wall of the capsule, the lamina externa. 



In all Urodeles studied, the planum basale is formed by a fusion of the 

 trabeculae in the middle line of the skull; and in some forms, this plate, 



