390 CHARLES CLIFFORD MACKLIN 



as well as man (Hertwig). It contains nothing but loose con- 

 nective tissue. It does not appear in Jacoby's figure, but the 

 optic foramen extends all the way to the interorbital septum, 

 and one may assume from this that the small isthmus of carti- 

 lage, which cuts off this small aperture from the optic foramen, 

 and which may be known as the commissura praechiasmatica 

 (figs. 1, 10, 14), has been developed between the 30 and 40 mm.. 

 stages. In my sections this commissure shows a rather younger 

 condition of cartilage than that found in the surrounding chon- 

 drocranium. The foramen praechiasmaticum evidently dis- 

 appears later, as it is not to be found in the osseous condition. 

 At the dorsal extremity of the optic foramen the lamina hypo- 

 chiasmatica is seen to pass over upon the dorsal root of the 

 ala orbitalis. 



A feature which I have not noticed in the description of human 

 primitive skulls, but which is described by Voit in the skull of 

 lepus, as the ala hypochiasmatica, is a small but strong crescentic 

 ridge which projects antero-laterally from the surface of the lam- 

 ina hypochiasmatica just ventral to the origin of the dorsal root 

 of the ala orbitalis, and which is continuous dorsally with this 

 root. It appears in figure 10 and may be seen from above as 

 a projection into the optic foramen (fig. 1). It presents a convex 

 ventral edge, and is separated from the surface of the interorbital 

 septum, lying within, by a distinct furrow containing only 

 connective tissue. In the Voit model of the skull of rabbit 

 this shows beginning ossification, but such is not the case in my 

 model, in which the ala presents a somewhat younger type of 

 cartilage, especially in the ventral edge, when compared with 

 that of the adjacent cartilage. 



Ventrally, as we have seen, the lamina hypochiasmatica is 

 continuous, medially, with the dorsal border of the interorbital 

 septum. From the sides of this septum the ventral roots of the 

 alae orbitales are seen to spring (fig. 1), and its cranialmost 

 edge, delimiting the orbitonasal fissure medially, passes over 

 directly upon the nasal septum. 



The interorbital septum (figs. 10, 11, 14 and 15) is narrow 

 cranially, being delimited dorsally by the roots of the alae orbi- 



