THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 609 



Ca 



arisen in the walls of the trunk in the region of the vertebral 

 column. Together they constitute a skeletal apparatus which under- 

 goes in the series of Vertebrates very profound and interesting 

 metamorphoses. Whereas it attains in the lower Vertebrates a 

 great development, it becomes in part rudimentary in Reptiles, Birds, 

 and Mammals. The part, however, which remains furnishes the 

 foundation for the facial skeleton. I begin with a short sketch of 

 the original conditions in the lower Vertebrates, especially in the 

 Selachians. 



As has been described in a previous chapter, the lateral walls of 

 the head-gut are traversed by the visceral clefts, of which there are 

 ordinarily as many as six in Sharks (fig. 331). The bands of sub- 

 stance intervening between 

 the clefts are called the 



membranous throat- or 

 visceral arches. They con- 

 sist of a connective-tissue 

 foundation invested with 

 epithelium, of transversely 

 striped muscle-fibres, and 

 of the visceral-arch blood- 

 vessels (see p. 571). Inas- 

 much as they have different 

 functions to fulfil, and con- 

 sequently acquire different 

 forms, they are distin- 

 guished as jaw-, hyoid, and 

 branchial arches. The most 

 anterior of them is the jaw-arch, which serves to bound the oral 

 opening. Following this, and separated from it by only a rudi- 

 mentary visceral cleft, the spiracle, is the hyoid arch, which is 

 connected with the origin of the tongue. Ordinarily this is followed 

 by five branchial arches. 



At the time when the membranous primordial cranium is con- 

 verted into cartilage, chondrification also takes place in the con- 

 nective tissue of the membranous visceral arches, thus producing the 

 cartilaginous visceral arches (fig. 331). These exhibit a regular 

 segmentation into several pieces, placed end to end and articulated 

 with one another by connective tissue. 



The jaw-arch is divided on either side into a cartilaginous palato- 

 quadratum (fig. 330 0) and a lower jaw (mandibulare). These 



39 



Fig. 331. Head of a Shark embryo 11 lines long. 



From PARKER AND BETTANY. 

 Tr, RATHKE'S trabeculse cranii ; PI. Ft, pterygo-quad- 



ratum ; Mn, mandibular cartilage ; Hy, hyoid 



arch; Er.l, first branchial arch; Sp, spiracle; 



Cl l , first branchial cleft ; Lch, groove under the 



eye; Na, fundament of the nose; E, eyeball; 



Au, auditory vesicle; C.I, C.2, (7.3, brain-vesicles; 



Hm, cerebral hemispheres ; f.n.p, fronto-nasal 



