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. Ido 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 
