70 



MAMMALIAN DESCENT. 



[Lect. III. 



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

 a special ingrowth of l:)one from the maxillaries and 

 palatines, is large and imperfect where the four plates 



-^.o 



Fig. 9. — Skull of an embryo Pig, 3 inches long, lower view, magnified 3J diameters. 

 al.n, aire nasi (snout cartilage); e.n, external nostrils ; ^ac, premaxillary ; mx, 

 maxillary ; d.g, dental groove (for tootli -pulps) ; ^Jrt, palatine ; p(/, pterygoid ; v, 

 vomer ; e.pc/, external pterygoid plate ; bs, basi-s plienoid ; g.f, glenoid fossa (hinge 

 cartilage for lower jaw) ; ti/, tympanic bone ; ob, os-bullie (additional tympanic 

 bone); bo, basi-occipital ; 2^-^(^j paroccipital process; oc.c, occipital condyle 

 (hinge for first vertebra or atlas); eo, 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 einl)iyo than 

 afterwards, but the l^one-cells are very thinly scattered 



