622 JOHN D. KERNAN, JR. 



that the latter change is pretty well completed when the former 

 has only just begun. The caudo-dorsal edge of the lamina hypo- 

 chiasmatica is the forerunner of the tuber culum sellae. There 

 are no traces of the middle clinoid processes. The plane of the 

 floor of the sella turcica forms an angle of 115° w^ith the upper 

 surface of the planum basale. It is from the angle formed by 

 their junction that the dorsum sellae rises. 



The lamina hypochiasmatica is limited cranially by a trian- 

 gular projection dorsad of the caudal extremity of the meseth- 

 moid. This will be described in connection with the ethmoid 

 region (figs. 1 and 8, 6). 



At the lateral edge of the lamina hypochiasmatica there are 

 paired little cylinders of cartilage which have their long axes 

 parallel to the plane of its surface (figs. 1 and 3). These are 

 known as the alae hypochiasmaticae (Voit). They are very well 

 developed in the rabbit. Macklin found them well developed 

 in the 40 mm. stage of the human and mentions them as having 

 never been described in man. Fawcett ('10), however, had fig- 

 ured them in the illustrations of his article on the development 

 of the human sphenoid bone, but described them as being con- 

 tinuous with the alae orbitales. The sections show in this em- 

 bryo (figs. 4, 5, 6) that they are independent of both ala orbitalis 

 and lamina hypochiasmatica, though they unite later with the 

 dorsal roots of the former (Fawcett) and the sides of the latter 

 (Macklin). 



The alae orbitales (figs. 1 and 3) have as yet no connection to 

 the sphenoid. They are S-shaped plates of cartilage which ex- 

 tend craniad and dorsad from the caudal end of the alae hypo- 

 chiasmaticae. Neither root is as yet formed, though there are 

 indications of the caudal root mesial to the caudal ends of the 

 alae hypochiasmaticae. There are as yet no indications of the 

 ventral, lateral and dorsal extensions found by Jacoby at 30 mm. 

 and Levi at 28 mm., which later unite the alae to the inter-or- 

 bital septum, to the ectethmoid, completing the optic foramen 

 and forming the roof of the orbit. Also the dorso-lateral exten- 

 sion, mentioned by Jacoby and Macklin as extending toward 

 the parietal plate and representing the commissura orbito- 

 parietalis of other mammals, is not present. 



