604 



Comparative Morphology of Chordates 



the skull becomes strongly convex on top and there may be a more or 

 less abrupt increase in convexity near the junction of the nasal region 

 and brain-case (Fig. 436JB). It is the forebrain which undergoes greatest 

 enlargement. Its expansion takes place laterally, upward, and back- 

 ward, so that it may come to overlie more posterior parts of the brain, 

 and, with extreme expansion, it (so to speak) presses strongly forward, 

 pushing the already bulging roof of the skull ahead of it so that the 

 region of the skull occupied by the frontal bones comes to lie to a 

 greater or less extent above the more posterior part of the nasal region 

 instead of behind it (Fig. 467). In connection with this change, the 

 eyes are shifted from their primitive lateral position and come to 

 be directed more nearly forward. Thus the mammal acquires a fore- 

 head — the region of the forward-bulging frontal bones, and a face. 

 In general, the animal whose main concern is to escape enemies 

 profits by having the eyes directed laterally so that it may have nearly 



iPOSTFRONTAL 



SQUAMOSAU 

 SPHENOID\ 

 FRONTAL 



SQUAMOSAL LACRIMA 

 QUADRAIOJUGAL NA SALr 

 PREMAXILLARY- 

 ABJJCULAR 



MAXILLA- 

 SURANGULAR 



LACRIMALv 

 NASAL' 

 MAXILLAs. 

 SFPTOMAXILLARY' 



PREMAXILLARY 



PREMAXILLARY 



Fig. 467. A series of fossil skulls (A-G) which probably represent fairly well 

 the phylogeny of the human skull. (After Romer: "Man and the Vertebrates," 

 University of Chicago Press.) 



