29) THE NASAL ORGAN IN AMPHIBIA—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 (fez pr) has developed and resembles that of 
the adult. The trabecula (¢), 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. 
