1 1] THE NASAL ORGAN IN AMPHIBIA— HIGGINS 11 



and the posterior lateral angle nearly meets the lateral margin of the tec- 

 tale (Fig. 5). Medially and behind, the cornu is continuous with the 

 planum basale, which has now extended laterally so as to form a partial 

 floor beneath the medial side of the olfactory sac. In all Urodeles the 

 produced posterolateral angle of the cornu supports the organ of Jacobson 

 (vomeronasalis). This relation and also that of the organ to the lateral 

 part of the tectale is of great aid in determining the homologies of the more 

 complex capsule of the adult. 



The antorbital process {pa) is now larger, forming a plate rather than a 

 bar, and its antero-lateral angle is directed forward as a blunt process which 

 nearly meets the posterior process of the cornu and also the lateral margin 

 of the tectale. 



In the nasal capsule of a 45 mm. larva, only a few modifications need 

 description. The capsule (Figs. 7, 8) has not increased in length, although 

 there has been an appreciable increase in width and depth. With the 

 greater development of the olfactory lobes there has been a corresponding 

 increase in the size of the cavum cranii (cc). The olfactory lobes extend 

 forward only as far as the caudal third of the nasal sac, so that the olfactory 

 foramina look obliquely forward from the antero-lateral angles of the brain 

 case. There is a gradual change during growth in the relative positions of 

 the nasal sac and the forebrain. In the earlier larva, brain and olfactory 

 organs overlap for about half the length of the nasal sac, while in the adult 

 the sensory structures are almost entirely in advance of the tip of the 

 olfactory lobes, approximating the anuran condition. 



The planum verticale {pv) of this stage is shorter than before but its 

 height is almost twice that of the 34 mm. larva; so that this stage marks the 

 beginning of the reduction of the verticale which is so much smaller in the 

 last stage to be described. 



The only other features to note in this stage are the broadening of the 

 tectale, which process has brought the foramen for a branch of the nasalis 

 internus on to the dorsal surface of the capsule and the extension of the 

 antero-lateral angle of the tectale, so that it now fuses with the postero- 

 lateral angle of the cornu, forming the first appearance of a lateral wall to 

 the capsule. The naso-lacrimal duct passes over this connection between 

 cornu and tectale, and empties into the olfactory sac just above the 

 anterior end of Jacobson's organ. 



Precartilage cells are abundantly distributed over the anterior end of 

 each olfactory sac, the anterior naris being terminal in all larval stages. 

 From these cells, the anterior cupola is formed; a process which involves 

 the shifting of the naris to its lateral position. 



In a larva near the end of metamorphosis (Fig. 9) many changes of the 

 capsule of the early stage have occurred. The general proportions of the 

 capsule remain unchanged, although there has been a further reduction in 



