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TEXT-BOOK OF EMBRYOLOGY. 



cavity, so that the brain and sense organs are practically enclosed. To this 

 capsule the term cartilaginous primordial cranium has been applied. (See 

 Figs. 170, 171, 172.) 



In the higher Vertebrates, chondrification is limited to the basal region of the 

 skull, while the side walls and roof are formed later by intramembranous bone. 



Meckel's cartilage 

 Malleus 



Incus 



Int. acoustic pores 

 Jugular foramen ' 



Subarcuate fossa 



Foramen (XII Nerve) 



Ala parva (sphenoid) 



Large occipital foramen Occipital 



(foramen magnum) (synotic tectum) 



Fig. 173. — Dorsal view of primordial cranium of human embryo of 80 mm. 



(3rd month). Gaupp, Hertwig. 



The membrane bones of the roof of the skull have been removed. Through the large occipital 



foramen can be seen the first three cervical vertebras. 



In the human embryo chondrification occurs first in the occipital and sphenoidal 

 regions, and then gradually extends into the nasal (ethmoidal) region. A little 

 later it spreads somewhat dorsally in the occipital and sphenoidal regions to form 

 part of the squamous portion of the occipital and the wings of the sphenoid. At 

 the same time cartilage develops in the embryonic connective tissue surround- 



