234 



THE DEVELOPMENT OF THE SKULL. 

 FIG. 71. 



[CHAP. 



rbs 



EMBRYONIC SKULL OP A FOWL DURING THE SECOND WEEK OP INCUBATION 



(THIRD STAGE) FROM BELOW. (From Parker. ) 



The figure is intended to shew the cartilaginous skull with its ossifications, 

 and also the splint bones which have commenced to be formed independently of 

 the cartilaginous cranium. The parts of the lower jaw and hyoid are omitted. 

 The following letters refer to the cartilaginous cranium and its ossifications. 

 pn prenasal cartilage, pa palatine bone (which has already become ossified). 

 pg pterygoid (also ossified), nc notochord visible in the post-pituitary space. 

 Both the notochord and the post-pituibary space would be in reality per- 

 fectly concealed by the plate of bone bt, but have been diagrammatically re- 

 presented as if they were visible through it. nc end of notochord appearing 

 beyond the occipital condyle. q quadrate, as alisphenoid. ic internal 

 carotid, ty tympanic cavity, psc posterior semicircular canal. 8 foramen for 

 passage of eighth nerve. 9 foramen for passage of ninth nerve, eo centre of 

 ossification for the occipital, so supraoccipital region, fm foramen magnum. 

 The following letters refer to the splint bones. 



px premaxillary. mx maxillary, pmx process inwards from the premaxillary to 

 form the maxillo-palatine. j jugal. qj quadrato-jugal. rbs sphenoidal 

 rostrum, bt basitemporal. '.,; 



