56 ILLINOIS BIOLOGICAL MONOGRAPHS [56 



tion of the antorbital in this stage, may be explained by the presence of the 

 muscularis process of the quadrate, and its attachment to the planum bas- 

 ale just ventral to the antorbital (Fig. 83). 



Just in front of the choana, which opens into the mouth slightly anterior 

 to the ethmoidal column, a longitudinal constriction of the nasal sac 

 partially divides it into two chambers; one medial and dorsal, lying upon 

 the lateral part of the trabecula, the other lateral and more ventral (it is the 

 beginning of Jacobson's organ), which lies to the side of and to some extent 

 beneath the trabecula. 



The olfactory nerves of the two sides now pass from the forebrain for- 

 ward and beneath the pons ethmoidalis to the nasal sac. Later, a chon- 

 drification ventral from the pons completely obliterates the fenestra 

 ethmoidalis and, forming the ethmoidal wall, it outlines the two olfactory 

 foramina in the anterior lateral angles of the cavum cranii. 



Anterior to the planum basale, the trabeculae diverge, enclosing a 

 wedge-shaped internasal space between them. At first more narrow, each 

 trabecula widens nearer its tip; and, just in front of the level of the naris, 

 bends abruptly downward to end near the inferior labial cartilage. This 

 vertical portion of the trabecula is apparently what Gaupp calls the super- 

 ior labial cartilage, and which he figures as a discrete element in both his 

 earlier and later stages. I do not find in my material any line of demarca- 

 tion between the trabeculae and the superior labial cartilages. 



I have had no material intermediate between the larval stage just 

 described and a young frog soon after metamorphosis. But it is easy to 

 see, however, that the changes undergone are about as follows; this account 

 agreeing substantially with that of Gaupp. 



As in a larva of Bufo, a tadpole of Rana approaching the end of meta- 

 morphosis, according to Gaupp, shows a decided reduction in the sagittal 

 plane of the anterior part of the nasal capsule. This reduction is caused 

 by a partial resorption of the earlier larval trabeculae, so that the definite 

 nasal capsule is formed by a development of the posterior parts of the 

 trabeculae, together with certain independently chondrified parts. 



Subsequent to the closure of the fenestra ethmoidalis, by the develop- 

 ment of a perpendicular ethmoidal wall, the planum verticale chondrifies 

 anteriorly, separating the nasal organs of the two sides and uniting the 

 planum basale in its median line to the planum tectale, which has grown 

 forward from the dorsal margin of the pons and the ethmoidal column. 

 The lamina externa forms the side wall of the capsule, and develops from 

 the lateral part of the tectale; it unites to the oblique cartilage, which chon- 

 drifies independently and lies diagonally across the nasal sac from the 

 lamina externa to the more anterior tectale. 



In the anterior part of the capsule, the greatest modification takes place. 

 The loss of the labial cartilages and the anterior parts of the trabeculae 



