THE ORGANS OF THE INTERMEDIATE LAYER OR MESEXCHYME. 609 



Au. 



Br.t 



Lch 



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 betw n 

 the clefts art- 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 

 )>y live branchial arches. 



At the time when the membranous primordial cranium is con- 

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

 iii ctive 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 (niandibulare). These? 



39 



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



From PARKER AND BETTANY. 

 Tr, RATHKE'S trabeculae cranii ; Pl.Pt, pterygo-qual- 



raium; Mn, mandibular cartilage; Hy, hyoi.l 



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



Cl l , first branchial clef c ; Lch, groove under the 



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



Au, auditory vesicle ; C.I, C.2, C.3, brain -vesicles ; 



Hm, cerebral hemispheres; f.n.p, fronto-i-.;t>jil 



process. 



