13] THE NASAL ORGAN IN AMPHIBIA— IIIGGINS 13 



ary of the external naris, along which the naso-lacrimal duct passes to the 

 olfactory organ (Fig. 45 Id). 



This stage represents the highest development of the cartilaginous 

 capsule in Amblystoma. The nasal organs are completely encased in 

 cartilage, except for the external and internal narial openings. In the 

 later stages studied, and in the early adult capsule, conspicuous gaps have, 

 formed through the process of resorption, giving way to the various mem- 

 brane bones that enclose these sensory structures in the adult. That the 

 completeness in the development of the capsule, which has been attained in 

 this stage, is in some way associated with metamorphosis, is 1 very evident, 

 and it probably represents the end of the larval period. 



The cartilaginous capsule of the adult Amblystoma (Fig. 10) results by 

 reduction and specialization of structures present in the stage just des- 

 cribed. Increase in the size of the sensory structures necessitates a capsule 

 of enlarged dimensions, although the proportions are relatively the same. 

 The planum verticale is reduced to a narrow bar which is relatively more 

 posterior in respect .to other capsular parts; its anterior ventral surface is 

 deeply excavate, the dorsal surface covering the intermaxillary gland to a 

 greater extent than before. 



Strikingly characteristic of the adult cartilaginous capsule is the large 

 five-sided gap in the roof which completely exposes the dorsal surface of the 

 nasal organ. This gap is the result of the further resorption which just 

 began in the last larval stage described. From its point of origin oppo- 

 site the verticale in the planum tectale, resorption has extended anteriorly, 

 laterally and posteriorly, until all that now remains of the complete carti- 

 lage roof is a slender bar of cartilage, the dorsal process of Winslow, uniting 

 the anterior cupola with the lateral wall of the capsule. Posterior to the 

 verticale, a narrow bar, a remnant of the ethmoidal column, passes over the 

 foramen olfactorius and connects with the posterior tectale which now is 

 reduced to a narrow band of cartilage. Lateral to the foramen orbito- 

 nasalis (fon) the tectale broadens slightly, and curving obliquely forward 

 forms a partial roof over the choana and lateral parts of the nasal sac 

 (Figs. 47, 48). More laterally this roof, better known as the lamina 

 externa (le), is pierced by three foramina, for a blood vessel and two branch- 

 es of the externus profundus nerves. 



The caudal extension of the cornu trabeculae is more posterior than 

 before, and a transverse section through the planum verticale shows the 

 posterior end of the organ of Jacobson lying between the cornu and the 

 lateral part of the tectale (Fig. 47). The fenestra infra-conchalis (fen ic) 

 is more elongate and contains the anterior end of Jacobson's organ (Fig. 48, 

 jo), which medially is partially covered by a part of the cartilage of the tectale 

 which supports the naso-lacrimal duct. From this, a cylindrical bar of 

 cartilage, the dorsal process, extends to the cupola, affording the only 



