70 



MAMMALIAN DESCENT. 



[Lect. hi. 



the upper region of the mouth, and which is formed by 

 a special ingrowth of bone from the maxillaries and 

 palatines, is large and imperfect where the four plates 



Fig. 9. — Skull of an embryo Pig, 3 iiiJ.^,. .,;._, lower view, magnified 3i diameters. 

 cd.n, ala3 nasi (snout cartilage); e.n, external nostrils ; ^Ja;., premaxillary ; mx, 

 maxillary ; cl.g, dental groove (for tooth-pulps) ; j)«, palatine ; pg, i)tt'rygoid ; v, 

 vomer ; cjig, external pterygoid plate ; bs, basi-splienoid ; g.f, glenoid fossa (hinge 

 cartilage for lower jaw) ; ty, tympanic bone ; ob, os-bulla; (additional tympanic 

 bone); ho, basi-occipital ; p.oc, paroccipital process; oc.c, occipital condyle 

 (hinge for first vertebra or atlas); co, exoccipital ; so, superoccipital ; f.m, 

 foramen magnum (great opening) ; IX. X. XII., holes for the exit of the 9th, 10th, 

 and 12th cranial nerves. 



should meet. It is less imperfect in the embryo than 

 afterwards, but the bone- cells are very thinly scattered 



