76 



COMPARATIVE ANATOMY 



in the course of further development, become situated in bays 

 or cavities within the head and enclosed by definite sense-capsules, 

 which take on close relations with the cranium, and thus are of 

 extreme importance in modifying the configuration of the skeletal 

 structures which are formed around them later. 



The relations of the visceral to the cranial skeleton, and those 

 of both to the primary metamerism of the head, must be taken 

 into consideration. Both cranial and visceral regions must have 

 been originally segmented, and each myotome at one time included 

 a ventral portion (lateral plate of the mesoderm) which enclosed a 

 corresponding section of the cranial ccelome, or <; head-cavity." 

 Later, however, the visceral region became relatively shifted to a 

 greater or less degree, especially in the anterior part of the head, 



FIG. 58. FIRST CARTILAGINOUS RUDIMENTS OF THE SKULL. 



C, notochord ; N, A, O, the three sense-capsules (olfactory, optic, and auditory) ; 

 PE, parachordal elements ; PR, primary pituitary space ; Tr, trabecuhe 

 cranii. 



so that its segments no longer corresponded to those of the cranial 

 region, which is in general more conservative as regards its 

 metamerism. Thus we find that the segmentation of the nervous, 

 muscular, and visceral parts of the head do not correspond with 

 one another. 



a. Brain Case (Neurocranium). 



The first cartilaginous rudiments of the primordial skull or 

 chondrocranium are seen in the form of an anterior and a posterior 

 pair of bars the trabeculce cranii and the parackordal cartilages 



