66 COMPARATIVE ANATOMY 



The portion of the skull which is situated along the main 

 axis in continuation of the vertebral column and which encloses 

 the brain is known as the hrain-hox or cranium, and is primarily 

 composed of cartilage. A series of cartilaginous arches arise in 

 serial order on the ventral side of the brain-case; these encircle 

 the anterior part of the alimentary tract like hoops, incomplete- 

 dorsally, and are distinguished from the cranial region as the 

 visceral skeleton. The latter stands in important relation to 

 branchial respiration, inasmuch as each consecutive pair of arches 

 encloses a passage (gill-slit), communicating between the pharynx 

 and the exterior ; this is lined by endoderm, and through it the 

 water passes in branchiate forms. The foremost visceral arch 

 bounds the aperture of the mouth, thus forming a firm support 

 for it, and giving rise to the skeleton of the jaws; the other arches 

 function primarily as gill-supports. Both cranial £gjd visceral 

 portions may become ossified later. 



Before the cartilaginous skeleton begins to be formed in the 

 embryo, the greater part of the head consists of a soft, mesoblastic 

 formative tissue, which gives rise to a membranous capsule around 

 the brain : the individual cerebral nerves can already be plainly 

 distinguished {memlranous stage, comp. p. 36). The three organs 

 of the higher senses also appear at a very early stage ; and these, in 

 the course of further development, come to be situated in definite 

 bays or cavities within the head, and thus are of extreme im- 

 portance in modifying the configuration of the skeletal structures 

 which are formed around them later. 



In the embryos of lower Vertebrates (e.g., Elasmobranchs) 

 more or less of the mesoblastic tissue which surrounds, isolates, 

 and supports these organs becomes divided up metamerically into 

 segments, so that a segmentation into somites (protoverteirce) occurs 

 in the posterior part of the head as well as in the body (comp. 

 pp. 8 and 36). The mesoblastic segments of the head, some of 

 which enclose cavities arising from the coelome (or the pre-oral 

 gut), consist of a tissue from which later become differentiated 

 all the supporting structures — including, of course, the skull, as 

 well as the muscles (myotomes). Without going into further 

 details as to the number and fate of these segments and their 

 relation to the cerebral nerves, concerning which there is con- 

 siderable diversity of opinion, it may be stated that the primary 

 segmentation of the part of the head posterior to the auditory 

 organ, in the region of the vagus and hypoglossal nerves, is at any 

 rate more pronounced than that of the more anterior part of the 

 head. 



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

 of both to the primary segmentation of the head, must also 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 mesoblast) 

 which enclosed a corresponding section of the cranial coelome, or 



