DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 
17 
skin. The valvular process of cartilage is seen here to be merely a part of the wall, 
and there is but a small space between this and the oblique floor-tract attached to the 
bottom of the septum. Two of the foremost of a series of glandular crypts ( gl.c .) are 
cut across, the larger of these is lodged between the side and floor. 
5th Section (Plate 3, fig. 5).—About six of the sections made at this part show a 
complete double nasal tube, for here the wall, valvular process, floor, and septum, are 
all confluent. Here the septum ( s.n .) is much thicker throughout; it still retains the 
winged growth above, and the valvular folds of the lining skin are very numerous. 
Here the thickened septum is of less vertical extent, and the thick ends of the floor 
drop from their point of union with the septum. There is a crypt {gl.c.) here under 
the valvular process, and another at the upper third of the wall, between it and the 
lining membrane. The nasals {n.) are wider here. 
6th Section (Plate 3, fig. 6).'—At this part the floor of the alinasal fold has again 
become free from the base of the septum {s.n.), which is of small vertical extent, but 
thick—thickest in the middle The upper alate enlargement is gone, but the fold of 
skin is still large, and runs out transversely. This is the last section showing the 
fold in the narial valve (n.v.), and there is a wide and hollowed space between it and 
the part forming the inner floor. That part is now tubular, the top of the tube {rc.c.) 
being sharp and fitting against the base of the septum. Besides the two pairs of 
glandular crypts {gl.c.), there is, in each tube of cartilage, an apparently glandular 
body; this is “ Jacobson’s organ” {j.o.), a structure which will appear in several of 
these selected sections.* 
Small points of bone are now apparent beneath the nasal canal, right and left, 
these are the premax diaries {px.) ; the nasals {n.) are still seen above. 
7th Section (Plate 3, fig. 7).—This section is behind the alinasal, in the fore part of 
the aliseptal region. The thickening which showed itself in the middle of the septum 
in the last section is now at the base; it is due to the fact that the intertrabecula is 
essentially a roundish and somewhat compressed bar of cartilage, which has shot up 
into a thick crest to join the roofs of the nasal capsule ( al.sp .). The middle valvular 
fold has now sunk down so as to be opposite the base of the septum; between it and the 
* Eor accounts of tlte structure and meaning of ttds part the reader is referred to the “ Bibliographical 
List. In my earlier papers on the skull my attention had not been directed to this part, and I was not 
then aware of the curious modification of the fore part of the skull caused by it. In the paper on the 
Pig s skull (Phil. Trans., 1874, Plates 28-37) the supporting cartilage is fi gured and described as the 
recurrent cartilage”—a name I shall retain. In the more recent papei’s on the skull of the Snake 
(Phil. Trans., 1878, Plates 27-33), and in that on the skull of the Lizard (Phil. Trans., 1879, Plates 
37-45), I have called the pair of Jacobson’s organs by Rathke’s term, namely, “nasal glands;” this, 
however, is a misleading term, as they have nothing to do with the nasal glands of Birds. In the Snake 
and Lizard these parts are protected by cartilage derived from the alinasal region, but they are actually 
encapsuled in a pair of curious bones; these are the paired vomers, which form the “ dish ” and the 
septomaxillaries that form its “cover.” The anterior paired vomers are very constant in Mammals; 
but I have not yet found the septomaxillaries in any member of this Class. 
MDCCCLXXXV. D 
