ILLINOIS BIOLOGICAL MONOGRAPHS 



THE NASAL CAPSULES OF THE URODELA 



AMBLYSTOMA PUNCTATUM 



Amblystoma was selected as the basis of these studies upon the nasal 

 organ of Amphibia, primarily because of the larger number of stages avail- 

 able, as well as on account of its intermediate position among the Urodeles. 

 I have studied and modelled seven stages of Amblystoma, which show the 

 successive steps, progressive and retrogressive, in the chondrification of the 

 nasal capsule of this Urodele. 



A larva 11 mm. long (Fig.l) shows but few features of a nasal capsule. 

 The trabeculae (/) extend forward along the ventral margin of the fore- 

 brain and in the region of the eye, each has developed a small dorsal crest, 

 so that in cross section it appears triangular. At this stage the trabeculae 

 do not unite, but each expands distally into a small rhomboidal plate, the 

 cornu trabeculae (ct), which supports the anterior end of the nasal organ. 

 In a slightly older stage, a few cartilage cells appear above the medial mar- 

 gin of the cornu trabeculae of the left side only (Fig. 41). These cells are 

 the beginning of a bar of cartilage, which develops along the medial dorsal 

 margin of the nasal sac, and is known in the literature as the ethmoidal 

 column, a term used throughout this discussion. 



In a larva 20 mm. long (Fig. 2), chondrification is much more advanced. 

 Slightly anterior to the internal nares, medial trabecular processes have 

 united in the middle line to form a broad planum basale (pb), which sup- 

 ports the anterior end of the telencephalon. This plate is broadly concave 

 dorsally, its lateral margins being elevated, the result of the trabecular 

 crests. Its posterior margin is straight and at right angles to the axis of the 

 skull, but it meets the medial margin of the trabecula in an oblique angle, 

 because of the converging lines of the latter. The anterior margin of the 

 planum basale passes into the expanded cornua trabeculorum, which lie in 

 practically the same plane as do the trabeculae. The cornua extend 

 laterally and each is concave upon its dorsal surface (Fig. 42) and supports 

 the anterior part of the nasal organ and the organ of Jacobson; while poster- 

 iorly it reaches about the level of the middle of the planum basale where its 

 posterior lateral angle terminates in a short caudal process. The antero- 

 lateral margin of each cornu is at about an angle of forty-five degrees to the 

 median axis of the skull. 



Dorsal and parallel to each trabecula and the median margin of the 

 cornu of either side is an elongate rod of cartilage, the further development 



