26 PROCEEDINGS OF THE ACADEMY OF [1889. 



as the septum nasi (Fig. 6, s. ?i.). Cribriform plate and lamina per- 

 pendicularis together form the mesethmoid. 



Jacobson's Organs, one on either side of the septum, are, as in the 

 vast majority of the rodentia, well developed in this embryo; and 

 as in the case of Lepus appear to be ensheathed by the backward, 

 extending, posteriorly free terminating palatine processes of the 

 snout, as seen in Figure 6 (re. c). These "organs" have been 

 described by Prof. Wiedersheim (Parker's translation) as being "a 

 paired accessory nasal cavity, which in an early embryonic stage be- 

 comes entirely separated from the nasal chamber, and which is 

 supplied by the olfactory and trigeminal nerves." They are sur- 

 rounded by the cartilages of Jacobson, likewise paired and of a scroll- 

 like form. Uniting with the antero-ventral aspect of the septum 

 nasi beneath, we are to note the ali-nasal cartilage on either hand, 

 encircling the external nostrils (e. n. and al. ».), while the ali-t-eptal, 

 one on each side of the septum on the dorsal aspect, run the entire 

 length and roof over the nasal chambers (Plate II, Fig. 5, al. sp.) 

 Large and of peculiar form, the aliethmoids form a striking feature 

 here, and may be seen both upon dorsal and ventral aspects. To 

 some extent they form the roof of each rhinal chamber, as well as 

 the sides and floor. Inferiorly, they become much expanded behind, 

 rounded, encircling submesial vacuities, as shown in Figure 6, (Plate 

 II, al. e.). 



Finally, turning to the ventral aspect of this minute endocranium 

 we see external to either recurrent cartilage, right and left of the 

 median ossified vomer (v), the inferior turbinal proper (/'. tb.), each 

 one in addition to its usual attachments in mammals of this order 

 is here connected with the septum nasi anteriorly, and terminates 

 behind in a teat-like process, extending somewhat further in 

 that direction than does the vomer. They swell at the middle of 

 their continuity, being convex externally and concave upon their 

 vomerine aspects. In a former paragraph, I have already alluded 

 to the " recurrent cartilages," so designated by Parker (re. c), and 

 these may now be seen upon this view. Especial attention is invited 

 to them as they are the essential capsules which enclose the organs 

 of Jacobson. To study them properly it becomes necessary to gently 

 press outwards the inferior turbinate. At present they are in car- 

 tilage entirely, thin, and of an elegant scroll-like form, being for the 

 most part open superiorly, and in contact with the vomer, mesially, 

 being in either ease outgrowths of the alee nasi. 



