12 ILLINOIS BIOLOGICAL MONOGRAPHS 12 



the length of the planum verticale, so that one-half of the entire capsule lies 

 anterior to this median vertical plate. The width of the verticale is also 

 reduced by one-half to accommodate the lateral development of the olfactory- 

 organs which now lie much closer together. The anterior surface is widely 

 concave, its dorsal margin overhanging the ventral, thus forming a partial 

 roof over the intermaxillary glands. 



The planum lectale {pt) now covers the entire dorsal aspect of the nasal 

 ac and extends from the region of the choana to the tip of the capsule 

 where it has united to the anterior margin of the cornu trabeculae, forming 

 the cupola (Fig, 9). This area is pierced by five small foramina. Four of 

 these are in an oblique quadrilateral, the fifth, much larger, lying behind 

 the others. Of the four, the anterior three foramina are for the branches 

 of the nasalis internus which are distributed to the dorsal part of the snout ; 

 while the lateral foramen passes a branch of the profundus from the capsule. 

 The larger posterior gap contains no nervous structures and apparently is 

 the beginning of the resorption of the cartilage roof of the capsule. The 

 medial surface of each capsule is marked dis tally by a small prenasal process 

 {pnp) at the base of which is the foramen nasalis internus {fni), above 

 described. On the dorsal surface of the capsule is a sixth small foramen for 

 a branch of the profundus of the fifth nerve, the result as before, of the 

 extension of the cartilage around the nerve. 



The antorbital process {pa) has now united to the lateral posterior 

 margin of the tectale, a condition foreshadowed by the close association of 

 these parts in the earlier stages, thus inclosing a large foramen between the 

 posterior margin of the tectale and the basal part of the antorbital. This 

 is the foramen orbito-nasalis {fon) into which the caudal part of the nasal 

 sac extends, and through which the nasalis internus of the fifth nerve enters 

 the capsule. Lateral to this foramen, the united elements of the antorbital 

 and tectale are directed obliquely forward, covering the lateral aspect of the 

 olfactory organ. A branch of the profundus nerve and a blood vessel 

 pierce this plate by two foramina just posterior to its connection with the 

 caudal extension of the cornu trabeculae (Fig. 9). 



As in the earlier stages, the organ of Jacobson rests upon the caudal 

 extension of the cornu which extends more posteriorly, and has partially 

 united to the fused tectale and antorbital. This union of the cornu to the 

 posterior tectale has resulted in the formation of a new foramen, the infra- 

 conchalis of Gaupp (Jen i c), through which the anterior part of the organ of 

 Jacobson extends to the laterally enveloping tissue (Fig. 46, jo). Thus 

 this organ rests upon a shelf formed by the cornu trabeculae, while its 

 anterior end extends through the foramen. This infra-conchalis is separ- 

 ated from the large lateral narial opening by the bar, which in the 45 mm- 

 larva connects the cornu trabeculae and the tectale. A deep groove occur- 

 in the lateral wall of the planum tectale which leads to the posterior bound- 



