320 CHARLES CLIFFORD MACKLIN 



Viewed from above (fig. 1) we note the entire absence of the 

 roof and the extremely rudimentary character of the sides of the 

 cranial vault. The eye meets with an irregular surface of vary- 

 ing depth, surrounded by a broken, ovoid contour, the smaller 

 end being ventral. This surface we recognize as the floor of the 

 primitive brain-case. Its dorsal half is made up of the future 

 posterior cranial fossa, a deep, bowl-like enclosure, the steep 

 sides of which slope rapidly down to an elongated opening in the 

 floor — the primitive foramen magnum. In the ventral wall of 

 this fossa is a trough-like space behind the basilar plate, flanked 

 by two rounded eminences, the partes cochleares of the otic 

 capsule, and terminated above by the horned, ridge-like dorsum 

 sellae, which forms a conspicuous object in the floor of the cra- 

 nium. Passing forward over this ridge a sideless pit, the hypo- 

 physeal fossa, comes into view, which marks the center of the 

 middle cranial fossa, and here, in the region of the body of the 

 sphenoid, the cartilaginous floor is very narrow. Lateral to the 

 corpus sphenoidale is a large, triangular gap in the floor and 

 sidewall of the brain-box, the apex of which meets the side of the 

 sella, while the ventral and dorsal borders are formed by the dorsal 

 border of the ala orbitalis and the cranio-ventral surface of the 

 otic capsule respectively. Forming a lateral, knobbed pro- 

 jection beneath the ala orbitalis is the relatively small ala tem- 

 poralis, and this is observed to lie ventro-caudal to the plane of 

 the above-mentioned triangular space, just as, in the osseous 

 skull, the greater wing lies below and in front of the plane join- 

 ing the lesser wing with the petrous portion of the temporal bone. 

 As will be seen later, this plane corresponds in a general way to 

 the situation of a primitive floor and side-wall of this region of 

 the skull, as found in the lizards. Two of the bones which will 

 later wall in this space, viz., the parietal and the squamo-tem- 

 poraUs, are as yet very rudimentary, while the third, or ala tem- 

 poralis, has only just commenced to ossify. 



We now pass forward, over the low ridge in front of the sella 

 turcica, known as the tuberculum sellae, and come upon the 

 plateau^like surface supporting the optic chiasma, which leads 

 laterally into the optic foramina. Ventral to this surface is the 



