THE SKULL. 



715 



mesoblast, called the investing mass, corresponding to that from 

 which the vertebrae are developed. 



From this arise two prolongations, which diverge and then 

 unite again, leaving an interval ; and the united portion becomes 

 once more divided into two processes, the trabeculce cranii. 



The mesoblast behind the interval receives the name of the 

 occipito-sphenoid portion ; the interval is the rudiment of the 

 sella turcica, which is occupied by the pituitary body. The part 

 of the mesoblast in front of this is called the spheno-ethmoidal 

 portion. 



FIG. 298. FIG. 299. 



FIG. 298. Basis cranii of a chick, sixth day. (Huxley.) 3. Trabeculse. 4. Pituitary 

 spaee. 1. Notochord. 5. Internal ear. 



FIG. 299. Longitudinal section through the head of an embryo of four weeks (KOl- 

 liker.) v. Cavity of cerebral hemisphere, a.no. Optic vesicle, z. Cavity of third ven- 

 tricle. TO. Cavity of raid-brain. A. Cerebellum, n. Medulla, o. Auditory depression. 

 t. Basis cranii. I'. Tentorium. p. Pituitary body. 



From the occipito-sphenoidal portion are developed the basi- 

 occipital together with the posterior part of the sphenoid. At the 

 sides of the medulla oblongata processes are sent up, which unite 

 round it and form the foramen 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 opisthotic, which is at the cochlea. 



