1 5] THE NA SA L ORGA N IN A MPHIBIA HIGGINS 1 5 



wards from the anterior cornu, just as in Spelerpes. At its posterior end a 

 small process, the beginning of the planum tectale, extends laterally and 

 then bends ventrally over the hinder end of the nasal sac at the level of the 

 choana; this planum tectale arising here as in Amblystoma from an out- 

 growth from the columna ethmoidalis. A small groove between the 

 anterior tip of the cornu trabeculae and the columna ethmoidalis allows for 

 the passage of the nasalis internus branch of the profundus nerve to the 

 internasal space (ins). 



An antorbital process (pa) arises from the latero- ventral margin of each 

 trabecula and extends laterally a short distance and anteriorly to a point in 

 line with the caudal margin of the planum basale. 



In a 38 mm. larva, the nasal capsule is similar in many ways to that of 

 the 45 mm. Amblystoma. The capsule has doubled in size and chondri- 

 fication has advanced in all parts, so that the olfactory organs are now 

 more completely protected. Each crista trabeculae has united to the 

 posterior end of the ethmoidal column, forming a lateral wall to the cavum 

 cranii, interrupted only by an oval foramen olfactorius (fo) for the olfactory 

 nerve. 



The planum basale (pb) resembles that of the earlier stage, but has now 

 completely lost the hinder process, so that the posterior margin is semi- 

 circularly excavate as in the 45 mm. Amblystoma. 



In a manner similar to that of the 25 mm. Amblystoma, an ethmoidal 

 bridge has formed uniting the columnae ethmoidales of the two sides. 

 This represents the beginning of the planum verticale which in the later 

 stage completely closes off the internasal space from the cavum cranii, now 

 in continuity by means of the circular fenestra ethmoidalis (Fig.51,feneth). 

 The dorsal surface of the rudimentary verticale is flat, continuous with the 

 dorsal margins of the ethmoidal columns; its anterior margin bears a 

 triangular cephalic process (ce p) projecting into the internasal space and 

 partially covering the intermaxillary gland, so that a sagittal section of the 

 verticale appears triangular. 



A lateral chondrification from the entire length of the ethmoidal column 

 now forms a broad planum tectale (/>/), which covers the riasal organ 

 throughout its entire length. Anteriorly the tectale has united with the 

 tip of the cornu trabeculae, forming a complete cupola (c), which is con- 

 tinuous with the verticale and is pierced only by a single foramen for the 

 nasalis internus of the profundus nerve. The lateral margin of the anterior 

 part of the tectale is separated from the lateral margin of the cornu by the 

 large oval narial opening, which lacks the deep fenestra narina of the 

 corresponding stage of Amblystoma; but similar to Amblystoma, the pos- 

 terior tectale unites by a small band of cartilage to the cornu, near its 

 posterior prolongation, which supports the organ of Jacobson. The naso- 

 lacrimal duct passes over this bar and is formed from two branches, which 

 come from the inner angle of the eye. 



