29] THE NA SA L ORG A N IN A MPHIBIA —HIGGINS 29 



In a 30 mm. larva (Fig. 26), some additional structures are present, 

 although the capsule differs considerably from any other Urodele. The 

 trabeculae are now united near their anterior ends by a planum basale 

 (pb), trapezoidal in outline, its posterior margin being one and one-half 

 times the length of the anterior. A median swelling upon the dorsal 

 surface of the planum basale, together with the trabecular thickenings 

 give the plate a bi-concave appearance, the olfactory lobes resting in the 

 concavities. This median swelling, more evident over the anterior half of 

 the planum and which resembles a similar structure in Cryptobranchus, 

 must be regarded as a rudimentary planum verticale, although the later 

 stage shows no further development of it (Fig. 64). Anterior to the plan- 

 um basale, the trabeculae continue forward a short distance; and, without 

 expanding into cornua so typical of other Urodeles, each trabecula, sepa- 

 rated from its mate by a wide internasal space, ends bluntly in the surround- 

 ing tissue. 



The beginning of the well-known fenestrated capsule of the adult 

 Necturus, covering the nasal organs, mentioned and figured by Wieder- 

 sheim (1877) and others, appears in this stage. Directly over the medial 

 margin of the nasal sac, and some distance from the trabecula is a small 

 bar of cartilage, the columna ethmoidalis (ce), which extends from a little 

 in front of the level of the anterior end of the trabecula back to the level 

 of the posterior margin of the planum basale. At no time is it united to 

 the trabeculae, and it recalls in origin and position this column in Amblys- 

 toma. A more posterior extension of this bar in the 33 mm. and 35 mm. 

 stages, in which it reaches nearly to the antorbital process, suggests that 

 here, as throughout the order, this bar chondrifies, first in the anterior 

 parts and then develops posteriorly; although here it never unites to a 

 crista trabecula, a structure entirely lacking in Necturus. Also in the 33 

 mm. and 35 mm. larvae I have observed cartilage cells along the lateral 

 margin of the nasal sac, some of which lie between the folds of the nasal 

 epithelium. These several areas do not seem to arise as a continuum, 

 but chondrify independently and later become connected to each other and 

 to the ethmoidal column to form the roof of the fenestrated capsule. 



The antorbital processes are now more like those of other Urodeles, 

 reaching forward a short distance toward the other parts of the capsule. 



In the last stage of Necturus studied, a larva 45 mm. long (Fig. 28), 

 the fenestrated nasal capsule (Jen pr) has developed and resembles that of 

 the adult. The trabecula (t), planum basale, and anterior extensions of 

 the trabeculae are much as before, differing only in size; while the planum 

 verticale, earlier prominent as the median ridge on the basale, is now 

 reduced to a small swelling on the anterior half of the basal plate. Accord- 

 ingly, the posterior half of the basale, which supports the olfactory lobes, 

 is slightly and broadly concave. 



