S("HULTE, SEI WHALK. 435 



it rather abruptly alters its shape, the left margin being displaced to make room for the descend- 

 ing aorta, while the right continues straight. Here the section becomes oval, the dorsoventral 

 diameter slightly exceeding the transverse. This form it retains until it begins to move ventrad 

 from the spine as it approaches the diaphragm. Here its section is circular and continues so 

 to its termination. Its diameter is 3.5 mm., and that of its open lumen is 2 mm. Here also 

 it shows the presence of low longitudinal folds. 



THE RESPIRATORY PASSAGES. 

 (Plates XLIX, Fig. 1, and LVI, Fig. 2.) 



Nasal fossa: (by John D. Kernan, Jr.). It is convenient for purposes of description to 

 distinguish between a respiratory passage and the olfactory region. The latter forms a sub- 

 spherical diverticulum from the dorsal wall of the respiratory passage. 



The respiratory passage is tubular, compressed from side to side, and is directed in a greater 

 part of its extent obliquely rostrad and dorsad. At the margin of the nasal bone its direction 

 changes, swerving dorsad almost vertically to the narial aperture. This passage has been 

 divided by Weber into proximal and distal portions, the nasal and naso-pharyngeal ducts, which 

 correspond approximately to the precerebral and subcerebral portions of de Burlet and other 

 authors. The rostral limit of the cerebral cavity in this fetus lies in the same transverse plane 

 as the crista semicircularis so that the whole olfactory region belongs to the pars subcerebralis. 



The precerebral portion is about half as long as the subcerebral portion. It is more com- 

 pressed from side to side and its lateral wall presents a somewhat complicated relief. This 

 depends upon the presence of the "Spritzsack," an oblique diverticulum which attains consider- 

 able depth above the level of the osseous paries. Toward the interior of the fossa the diverti- 

 culum diminishes in depth, becoming a shallow furrow. It is bounded by two prominent folds, 

 which with the diverticulum have a spiral course. The rostral fold at the narial aperture is 

 broad and forms the lateral lip of the naris, here lying dorsal to the diverticulum and forming 

 its roof. As it is traced into the respiratory passage it diminishes in height and terminates by 

 passing upon the septum. It thus describes a spiral from the lateral to the mesal wall of the 

 passage, crossing the rostral paries in a ventro-mesal direction. The second fold belongs to 

 the caudal and lateral walls, upon which it descends in a semicircular course, and in its whole 

 course occupies the concavity of the first fold. Toward the narial aperture it forms the floor 

 of the "Spritzsack." It owes its prominence and direction to the cartilage cupularis and forms 

 a cushion upon which the first spiral fold is molded, and against which it becomes firmly coapted 

 when pressure is made upon the narial region from without. The arrangement of these folds 

 would thus seem to secure the effectual closure of the respiratory passages when the anima 1 

 is submerged. The furrow which separates these two folds at its termination upon the septum 

 forms a shallow fossa, which corresponds in position to the open groove in which de Burlet 

 recognized the rudiment of Jacobson's organ. Ventral to the nasal bone the lateral wall of the 

 respiratory passage is concave, save for a longitudinal ridge situated midway between roof 

 and floor. This has a length of 5 mm., a breadth of 1.5 mm., and a height not exceeding 1 mm. 

 It is the expression of a ridge in the cartilaginous wall of the nasal fossa, and is probably the 

 equivalent of the naso-turbinal. Its ventral margin is slightly undermined by a corresponding 



