THE SKULL. 



719 



round it and form the fo rain on magnum. Laterally the mesoblast 

 envelops the auditory vesicles and forms the side portions of the 

 occipital bone. 



In the cartilaginous antecedent of the temporal bone there are 

 three centres of ossification the epiotic, which develops the mas- 

 toid process; the prootic, which is in the region of the superior 

 semicircular canal ; and the opkthotic, which is at the cochtea. 



FIG. 298. 



no 



Fi... 2i'S. - Hasis eranii of a chick, sixth day. (Huxley.) 3. Trabeculw. 

 4. Pituitarv space. 1. Notochord. 5. Internal ear. 



Fiu. 299. Longitudinal section through the head of an embryo of four 

 week^KollikerV c. Cayity ot' cerehral hemisphere, n.no. Optic vesicle. 

 ;. C'avity of third ventricle, in. Cavity of nml-hrain. h. Cerebellum, n. 

 .Medulla, i). Auditory depression, t. Kais cranii. ('. Ten tori um. p. 

 Tituitary l>ody. 



The spheuo-ethmoidal portion develops the anterior part of the 

 sphenoid together with the ethmoid bones and the cartilage of the 

 septum of the nose, the first arising from the back part is developed 

 from mombrano. The tru been he a re carried forwards, and bending 

 down at the nasal depression form the lateral nasal cartilages and 

 the anterior part of the septal cartilage. 



The face is developed in connection with ridges known as the 

 finceral folds or arches, between which are a number of clefts, the 

 visceral clefts. 



The eyes and the openings of the nose are in the face; whilst 



