606 JOHN D. KEENAN, JR. 



crista transversa cranially, which marks its junction with the 

 sphenoid body. At its caudal end it divides into two branches 

 which join on either side the paTtes laterales of the occipital re- 

 gion. Craniad to the occipital region it has on either side the 

 large oval masses of cartilage which make up the otic capsules. 

 These, with the plates of cartilage which extend them dorsally 

 form the side walls of the skull in the otic region. 



Craniad to the otic region, the basal plate joins the sphenoid 

 body. Side walls are here lacking. In their place we see two 

 processes on each side, caudal and ventral, the alae temporales; 

 cranial and dorsal, the alae orbitales. From the crista trans- 

 versa a prominent dorsum sellae springs upward. 



With the vertebral column in the vertical position, the planum 

 as it extends ventrad, rises in level. The slope is not uniform. 

 Caudally in the occipital region, and cranially approaching the 

 sphenoid, there is a fairly sharp rise. Between these sloping 

 areas is a region which is horizontal. Thus the basal plate in 

 its sagittal extension shows two angles. The first, at the junc- 

 tion of the caudal inclined area and the horizontal portion, points 

 dorsad. The second, at the junction of the ventral inclined area 

 and horizontal portion, points ventrad.. In his series of models 

 of human chondrocrania Levi demonstrated that a change of 

 level of the basal plate (the vertebral column being held verti- 

 cal) has a great influence on the course of development of the 

 skull. His youngest embyro (13 mm.) showed the basal plate, 

 in so far as it was developed, horizontal and straight. A 17 mm. 

 embyro showed a condition much like that described above for 

 the 20 mm. a horizontal middle region joined at either end by 

 inclined areas which form angles with it. His oldest embyro 

 (28 mm.) showed a basal plate with almost vertical areas 

 caudally and cranially, having a portion less steep between them. 

 Thus the basal plate in this embryo showed the same angles as 

 that of the earlier embryo and in addition the plate as a whole 

 had here changed its level. 



Consideration of the conditions found in all these embryos 

 would appear to show that two processes are involved in bring- 

 ing the basal plate to its final shape and level. The first is the 



