102 Trans. Acad. Sci. of St. Louis. 



lie the caudal part of the main nasal sac with the posterior 

 naris and also the caudal part of the organ of Jacobson. 



The olfactory window (Fen. ol.) has resulted from the 

 union of the rod (Col. eth.) with the dorsal corner of the 

 Crista trabeculae ; its ventral boundary is the trabecula 

 while the cephalic limit is made by the union of the latter 

 and the rod. The distance between the middle of the rod and 

 the trabecula, (i. e., the height of the olfactory window) has 

 increased relatively, the rod suffering a bend, convex dorso- 

 laterally. This appears to be due to the growth dorsally of 

 the nasal sac to which the rod is now indirectly fixed through 

 the lamina cribrosa to be described presently. Through 

 the window passes the olfactory nerve ; in its ventral and 

 cephalic corner is the end of Jacobson' s gland. This does 

 not enter the cranial cavity but presses against a membrane 

 which fills the window and which is traversed by the olfac- 

 tory nerve. The caudal boundary of the window is the 

 anterior free edge of the trabecular crest and also to some 

 extent, the ventrally turned end of the rod (Pr.unc.) which 

 has grown behind the olfactory nerve. Just as the nasal 

 sac itself, so the olfactory window has an oblique position and 

 forms an obtuse angle with the sagittally directed Crista 

 trabeculae. 



The rod (Col. eth.) is now considerably flattened and sends 

 an irregular edge over the nasal sac into the perirhinal mem- 

 brane. The rods of opposite sides are in this stage connected 

 across the middle line by a bridge of cartilage which I shall 

 call the Ethmoidal Bridge (Eth. br. ). This is formed, in part 

 at least, by extension and fusion of the ethmoidal processes 

 (Pr. eth.) of Stage II. The bridge passes in front of the 

 brain, partially separating the cranial cavity and internasal 

 space, and lies in a transverse plane cephalad of the anterior 

 margin of the anterior trabecular plate. Between these parts 

 and the bases of the rods (Col. eth.) laterally is the eth- 

 moidal window (Fen. eth.) filled with membrane; part of 

 it is beginning to chondrify. 



The medial nasal process (Pr. n. med.) has grown forward 

 and widened in the dor so-ventral direction in the perirhinal 



