156 EMBRYOLOGY OF THE LOAYKi; VERTEBRATES CH. 



richly vascular and the renewal of the water in contact with their 

 surface is provided Tor l>y a well-developed muscular mechanism by 

 which they are sharply liicked from time to time, or, in early stages, 

 liy rich cifiation of their surface as in the Frog (Assheton, 1896) or 

 fobranchus (Smith, 1912). They are as a rule merely temporary 

 organs. As the respiratory function comes to be sufficiently per- 

 formed by other organs their circulation becomes sluggish, their 

 tissues moribund. They become invaded by leucocytes and eventually 

 undergo complete atrophy. In Protopteras distinct vestiges persist 

 t>r a prolonged period while in various Urodeles they remain func- 

 tional throughout life. 



The external gills, highly vascular and projecting freely into the 

 surrounding medium, present tempting objects for attack by other 

 organisms. They are therefore extremely liable to injury, and cor- 

 related with this they present a high power of regeneration. In 

 correlation also with the same fact we find that they tend to be 

 eliminated from development in certain members of groups which are 

 as a whole characterized by their presence. Such is the case in the 

 Amphibia where they are characteristic of the group in general but 

 where in particular cases they are reduced (Hyla arbor ea) or com- 

 pletely absent (Boiribinator) although we must believe they were 

 present in the ancestors of these forms. 



This tendency for the external gills to become eliminated from 

 development in the process of evolution raises the interesting morpho- 

 logical question : were External Gills at any period more widely dis- 

 tributed amongst Vertebrates than they are at present ? And, if so, 

 are their vestigial representatives still to be found in any cases where 

 they no longer develop as functional respiratory organs ? 



This interesting problem, which offers an inviting field for research, 

 has not yet had sufficient attention devoted to it. Even if it were the 

 case that external gills once existed in the ancestors of forms in 

 which they are no longer present as functional organs there is always 

 the possibility if not probability that their disappearance has been so 

 complete as to leave no observable trace. Nevertheless such vestiges 

 might persist and are worth looking for. 



Under these circumstances it is of interest to note that already 

 certain structures are known which are interpretable as vestiges of 

 once-present external gills. Thus in Gymnophiona what appear to 

 be transient rudiments of mandibulsir and hyoidenn external gills 

 make their appearance during development (Fig. 87, B). Again in 

 the case, of the Mandihular and Hyoid arches of Urodeles, on which 

 no functional external -ills develop, Driiner (1901) has found what 

 appear to be vestiges >f the muscles of external gills. A-'ain in 

 the larvae of various Urodeles there occurs in connexion \\ith 

 each mamlil.iilar arch a curious styliform projection known as the 

 balancer, from the fact that the 1,-irva ha lanc.es itself upon them as 

 upon a pair of limits (V\^. HS, //). Kach of these has a vascular loop 

 within it and it in l:n-i appears to be the modified external ^ill of 



