THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 607 



which corresponds to the sella turcica, and has been caused by the 

 formation of the hypophysial pocket from the oral sinus and by its 

 growing through the membranous basis of the cranium toward the 

 infundibulum of the brain. Rather late there is also formed, as the 

 floor of the sella turcica, beneath the hypophysis, a thin cartilaginous 

 plate, which is pierced only by the holes for the internal carotids. 



After the base of the cranium has been developed, the process of 

 chondrification involves the side walls and at last the roof of the 

 membranous primordial cranium, precisely as the halves of the 

 neural arch grow out from the body of the vertebra and finally 

 terminate in the dorsal spine. 



In this manner there is developed around the brain in the case of 

 the lower Vertebrates, in which the axial skeleton remains in the 

 cartilaginous condition throughout life (fig. 330), a closed, tolerably 

 thick-walled capsule, the cartilaginous primordial cranium. 



In the higher Vertebrates, in which to a greater or less degree 

 processes of ossification occur later, the primordial cranium attains a 

 less complete development, as is shown by the fact that its walls 

 remain thinner, and indeed acquire at some places openings, which 

 are closed by connective-tissue membranes. In Mammals the latter 

 condition occurs very extensively in the roof of the skull, which 

 becomes cartilaginous only around the foramen magnum, whereas in 

 the region in which afterwards the frontal and parietal bones are 

 located the cranium remains membranous. The cartilage attains a 

 greater thickness only at the base of the cranium and in the regions 

 of the olfactory organ and the membranous labyrinth, where it gives 

 rise to the nasal and ear capsules. 



For the sake of better orientation, it is useful to distinguish in the 

 primordial cranium different regions. There are two different prin- 

 ciples of division that may be employed in this connection. 



Following Gegenbaur, one can divide the primordial cranium, in 

 accordance with its relation to the chorda dorsalis, into a posterior 

 and an anterior portion. 



The posterior region reaches up to the dorsum sillae and encloses in 

 its basal portion the chorda, which in Man enters into it from the 

 odontoid process through the ligamentum suspensorium dentis. The 

 anterior portion is developed in front of the pointed end of the 

 chorda out of Eathke's cranial trabeculse. Gegenbaur designates 

 the two as vertebral and evertehral regions (for which Kolliker 

 employs the names chordal and prechordal) ; he shows that the 

 vertebra^ region must be, on account of its relation to the chorda, the 



