36 Mr. IT. G. Scclcj o?i the Origin 



])V potential growth — if ancestors of sucli animals arc con- 

 sidered ever to have had sucli bones. 



This ])eing, as I suppose, tlic mode of origin and phm of 

 grovvtli of tlie neural arches of the skull, I turn to explain the 

 inferior arclies. 



\n sliarks the head is singularly instructive in tlie relation 

 of tlie jaws to the skull ; for tiierc thej are seen to be free 

 structures wliich are merely appended to the brain-ease. 'J'liis 

 condition, permanent in the shark, is embryonic in what 

 are called higher Vcrtebrata. 



The jaws are the entrance to the digestive canal ; and tliere- 

 fore we must anticipate that they will be surrounded witli 

 bones which are the representatives of those which encoin]);iss 

 the digestive organs in the region of the vertel)ral column, 

 vi/. of ribs. Prof, llathke, describing tlui embryonic develoi)- 

 ment of the jaws in serpents, records that " that part of the 

 investing mass of the notochord in which the basisphenoid is 

 develo])ed in many animals sends out a ' ray ' or band down- 

 wards on each sid(^:, which presents a remarkable similarity to 

 a ril), not only in its mode of origin, but in its original posi- 

 tion and form," " ]>ut very early there grows out from near 

 the uj)])er end of the ray a long thin process, which passes off 

 at an obtuse angle to it, and applies itself to the inferior wall 

 of the futnre l)rain-case." Now this condition is that of an 

 ordinary I'lh of a hsh. There is a long rib, as in mammals ; 

 but near its junction with the vertebra it gives off by artieu- 

 l.ation a long thin epiplcural element, homologous with that 

 of Croeodiles, Ihitteria^ Birds, t^c. ; so that I see no reason to 

 doubt that the jaws are developed primarily as one rib, the 

 epi])leural elements of the two sides being directed forward 

 and meeting in the middle line, so as to form the ])alate, and 

 the oi-dinary pleural elements being directed downward so as 

 to meet and enclose the digestive tulje below. The ribs of 

 fishes are sim])le; but in re])til(!S and birds and mannnals tliey 

 become segmented; and there appears to be no limit to the 

 number of parts which may be included, while the degree of 

 ossification is various. In some animals there are five parts. 



In th» serpent the e])ij)leural element becomes segment(>(l 

 into the ])t('rygoid, palatine, and maxillary bones ; while the 

 rib itself is divided into the quadrate bmie ])roximally, tlien 

 the articular bone, and then the elements of the lower jaw, 

 which sui-round the cartilage and nmy number as many as 

 five. The cranial representative of the rib always articulates 

 with the squamosal bone. 



It must at once occur to any one to ask, if the cranium 

 eousi.sLs of three segments, and oidy the middle one developes 



