14 



NATURAL HISTORY OF VERTEBRATES. 



strips or plates, which eventually unite with each other. Fig. 14, which represents 



a section of the thoracic region of a crocodile, 

 shows the intimate relation that these structures 

 have to each other, to which reference will af- 

 terwards be made. The sternal and vertebral 

 halves of the ribs (r and sr) may be somewhat in- 

 dependent of each other, and the latter may be 

 locked together by overlapping imcinate processes. 

 After the fusion of the cartilaginous strips into 

 the sternum, the. latter frequently becomes in the 

 mammals secondarily segmented into bones to the 

 intervals between which the ribs are attached, 

 (Fig. 15). 



The snakes are characterized by the absence 

 of a sternum, which, like the. absence of the 

 limbs, is presumably secondary. A similar ex- 

 planation must be given for the absence of the 



sternal ends of the ribs in Amphibia, where the sternum itseK is present and closely 



connected with the shoulder-girdle. 



Fig. 15.— Shoulder-girdle of a field mouse ; 

 ac, acromiou process ; c, coracoid process ; 

 cl, clavicle ; eps, episternum ; sc^ scapula ; 

 sr, sternal; v, vertebral rib ; x, xiplioid car- 

 tilage. 



The Skull. 



Just as the vertebral column is derived from the skeletogenous tissue surrounding 

 the notochord, so the groundwork of the skull is derived from similar tissue, which 

 lies on either side and in front of the cephalic end of the notochord. But the 

 demands on this tissue are of a very different character in the cephalic end of the 

 body. The central nervous system in this region swells out into the brain ; for tliis a 



special protective capsule is required, which, of course, 

 adapts itself in foi"m to its contents. No segmentation 

 like that in the vertebral column occurs in this chon- 

 drocranium, which is further modified by protecting 

 the auditory organ behind, the olfactory in front, and, 

 to a less extent, the eye in the middle. These organs 

 thus mark out regions in the chondrocranium (the 

 olfactory, orbital, and auditory) while a fourth region, 

 the occipital, is that which establishes the union with 

 the vertebral column. 



Fig. 16 indicates how part of the skeletogenous 

 tissue {p, the parachordal tracts) lies at the side of 

 the notochord, while two other tracts. (the trabeculae) 

 are entirely prechordal, and thus not comparable to the 

 tissue around the notochord. The parachordal tracts 

 soon coalesce with the auditory capsules, and tend to 

 grow upwards round the brain, as do the trabecular 

 tracts further forwards. The latter also contribute to 

 the formation of the olfactory capsules, so that a cartilaginous box for the accom- 

 modation of the brain results, the walls of which are, however, in most forms, 

 deficient in certain places (fontanelles). It is this chondrocranium which furnishes the 

 substratum in or on which the cranial bones are developed. Even in its notochor- 



FiG. 16.— Head of embryo dog-flsh ; a, 

 auditory capsule ; c, trabecular cor- 

 nua ; c/, visceral clefts ; 6, brancbiie; 

 e, eye ; i, infundibulum ; o, olfactory 

 organ ; n, notochord ; p, parachordal 

 cartilage ; s, spiracle ; t, trabecula. 



