80 



COMPARATIVE ANATOMY 



both phylogenetically and ontogenetically. These perichondral 

 bones extend into the cartilage and many even entirely replace 

 it (" endochondral " bones) ; they may therefore be called replac- 

 ing or substituting bones (autostoscs) (cf. p. 45). It must, 

 however, be borne in mind that the stage of development at 

 which the different bones appear gives no accurate basis for 

 phylogenetic speculations : bones which correspond in position as 

 well as in other respects and thus appear to be homologous, may 

 be developed in different ways in different Vertebrates. 



The development of bones starts from so-called " centres of 

 ossification," of which there may be several in a single resulting 

 bony territory. An earlier or later fusion of these centres or 

 even of entire bones, leads to a reduction in number; while on 

 the other hand, " supernumerary bones " may occur owing to 

 the absence of fusion between separate centres. 



!>. The Visceral Skeleton (Splanchnocranium). 



The primarily cartilaginous visceral arches, which are developed 

 successively from before backwards in the lateral plates of the 



mesoderm, encircle the anterior section 

 of the alimentary canal, and are situ- 

 ated in the interbranchial septa (Figs. 

 62 and 63). The branchial apparatus 

 at first lies beneath the hind-brain, and 

 the arches are thus included under the 

 cranial skeleton, of which, however, they 

 are mostly genetically independent. 

 Later, owing to unequal growth, most of 

 the gill-sacs become relatively shifted 

 backwards so as to be situated in the 

 region of the trunk. The visceral arches 



62. DIAGRAMMATIC are always more numerous (in some 

 cases there are as many as nine) in 

 forms which possess gills than in higher 

 types (Amniota), in which their number 



pharynx, enclosed "by the gradually becomes reduced from behind 

 visceral skeleton ; 1 to 4, forwards : they may, moreover, undergo 



the individual elements . *..' . ,> , 



FIG. 



TRANSVERSE SECTION OF 

 THE PRIMORDIAL SKULL. 



a change of function, certain of them in 

 some cases taking on definite relations 

 to the auditory organ, larynx, and 

 tongue. 



The most anterior arch arises 

 first and serves as a support for the 

 walls of the mouth which receive their nerve supply from the 

 trigeminal : it is distinguished from the other or post-oral arches 

 as the mandibular arch (Fig. 63). The post-oral arches serve 



composing each visceral 

 arch, which is united with 

 its fellow by a basal piece 

 (Cp) ; Tr, trabeculee, which 

 enclose the brain (C) ven- 

 trally and laterally. 



