734 SNOUT OF FCETUS OF MONOTREMES, 



area formed by the ventral edge of the septum nasi where it descends to 

 become intercalated into the nasal floor. 



Fig. 12. Left lateral aspect of Model iii. The interrupted character of the 

 cartilaginous boundary of the anterior narial aperture is apparent. The* 

 posterior aperture-cartilage {cap. p.) is continuous behind and internally 

 with the alinasal, and just behind this attachment the alinasal is continued 

 backwards for some distance as a tapering conical cartilage visible in fig, 9 

 (c.p.), but hidden here under cover of the premaxillary splint, _p.T". Other 

 letters as elsewhere. 



Plate xl. 



Figs. 13-21 represent a series of coronal sections through the region of the 

 snout of the younger foetal Ornithorhynchus. They may be collated with the 

 figures of Model i. 



Fig. 13 represents the 20th section of the series, the os carunculae having 

 first appeared in the 12th section. The section cuts the tips of the rostral 

 prolongations of the marginal expansions of the great plate of cartilage which 

 behind forms the nasal floor. The prerostral notch lies between the tips of 

 the cartilages. 



In fig. 14 the premaxillary trabeculse continuous above with the os carun- 

 culse are passing ventrally through the prerostral notch to join the rest of 

 the premaxillffi. 



In fig. 15 we reach the plane of continuity of the median septal bar (s.«.) 

 and the cartilage of the nasal floor, the section passing just in front of the 

 septal fenestra. The os carunculae is now tapering away posteriorly. In 

 figs. 14-16 the epidermis over the caruncle is somewhat thickened, though 

 not excessively. It is stained yellow by the picric acid which was used as a 

 counter-stain with the hamiatoxylin. A study of sections in front of those 

 here figured shows that the cornification is most intense in the epidermal 

 layers which correspond to the stratum lucidum. 



Fig. 16 passes through the plane of the anterior nares and the septal 

 cartilaginous fenestra. Here we can distinguish dorsal and ventral lamellae 

 of the body of the premaxilla as well as the true palatine processes. 



Fig. 17 passes behind the cartilaginous septal fenestra, and shows the 

 continuity of the wide ventral portion of the septum nasi with the cartilage 

 of the nasal floor. The section is slightly in front of s.n'. in fig. 3. The 

 upper and lower valvular folds are here cut across. 



Fig. 18 is a little in front of the plane of the partial coronal section of 

 Model i., seen in fig. 3 in the right side of the figure. The anterior end of 

 the cartilaginous capsule of Jacobson's organ is cut across. The marginal 

 expansions of the nasal floor cartilage are now no longer continuous medially 

 with the alinasal. 



Fig. 19 is practically in the plane marked by n.p.c. in fig. 3. It passes 

 through the naso-palatine canal and the opening into the latter of the efferent 



