v VISCERAL AECHES 319 



segmented condition (Zepidosiren, Amniota) and no trace of segmenta- 

 tion appears during ontogeny. 



In Elasmobranchs (Dohrn, 1884) chondrification begins on each 

 side and then spreads dorsally and ventrally. Segmentation takes 

 place first into a dorsal and ventral half and later each of these 

 segments again. The gill rays develop independently of the hoop 

 and only come into contact with it later. 



The hyoid arch corresponds closely with the branchial arches in 

 its mode of development. 



The arches so far dealt with — branchial and hyoid — having to 

 do primarily with the function of branchial respiration show their 

 typical development in Fishes. With the disappearance of this func- 

 tion they become degenerate. This degeneration makes itself manifest 

 in (1) reduction of segmentation, (2) tendency to fusion between 

 successive arches and (3) reduction in number from behind forwards. 



Thus in a Newt four cartilaginous branchial arches make their 

 appearance but they are for a considerable period continuous dorsally 

 and ventrally with their neighbours in the series, and they develop 

 only one joint upon each side i.e. the half-hoop consists of two segments 

 instead of four. In a Lizard only two cartilaginous branchial arches 

 make their appearance, and in a Bird only one. , 



The hyoid and the anterior branchial arches have probably been 

 saved from complete disappearance in the higher Vertebrate by the 

 fact that they have taken on important functions in connexion 

 with the tongue and have become specialized in accordance therewith. 

 Thus in the case of the frog tadpole there is found, when the 

 branchial apparatus is at the height of its development, a continuous 

 cartilaginous hyobranchial skeleton, in which may be recognized 

 parts corresponding to hyoid arches, copula between these, and 

 4 pairs of branchial arches continuous ventrally. At the time 

 of metamorphosis this becomes greatly modified to give the adult 

 condition (Gaupp, 1894) : the mid-ventral portions become greatly 

 expanded to form a flattened plate — the so-called " body of the 

 hyoid " : the hyoid arch becomes an elongated slender rod which 

 serves to suspend the apparatus from the skull : the branchial arches 

 disappear except the ventral end of the second which persists as a 

 stump (" Postero-median process "). 



Mandibular Arch. — The usually accepted idea of the mandibular 

 arch is to regard it as a half-hoop shaped cartilage resembling the 

 other arches, to which is added a forwardly projecting outgrowth — 

 the palato-pterygoid bar — which forms the primitive upper jaw 

 skeleton. In actual ontogeny there is always a less or greater 

 amount of departure from this general scheme. 



In the Amphibians and Lung-fishes the hoop-like character of 

 the main portion of the arch has been most completely retained. 

 Here (Fig. 155, A) the arch develops on each side as a curved bar 

 of cartilage— a mid-ventral copula having been detected in certain 

 cases. The cartilage soon becomes completely continuous at its 



