450 EMBRYOLOGY OF THE LOWER VERTEBRATES OH. 



evidence appears to be conclusive that they are truly homologous 

 throughout. 



It has been shown that there is a tendency for the External 

 Gills to become eliminated as e.g. in various Anurous Amphibians : 

 it has been shown further that in some of the main groups of Verte- 

 brates in which they do not occur their disappearance may be 

 accounted for by the evolution of a new physiological substitute 

 the vascular surface of the yolk-sac. 



Having regard to these facts and to the relatively archaic 

 character of the groups in which they actually occur the conclusion 



is considered justifiable that such external 

 gills are organs of high antiquity in the 

 Vertebrate stem. Further, from their dis- 

 tribution upon the various arches it is in- 

 ferred that in all probability an external 

 gill was once present upon each visceral 

 arch. But it has also been shown to be 

 probable that the series of visceral clefts 

 and therefore of visceral arches was 

 formerly more extensive, extending farther 

 back along the body than it does in exist- 

 e.g.t M ing Vertebrates. It is therefore concluded 



that in an earlier phase of its evolution 

 the phylum whose modern representatives 

 we call Vertebrates was characterized by 

 FIG. 204. Left side of head the possession of a series of external gilla 

 ^"l^,^ ^tending Awards beyond the limit 

 .lays pn-viou>iy, :i jien- of reached by the branchial region of exist- 



skin froin the branchial region ing Vertebrates. 



of another embryo had been r> f anpU ovtarnnl mile QVP rntpnHal 



grafted. ( After Kk.nau, 1913.) \ aie potential 



organs of support as shown by the 



...1 mils (auto- "balancers" of Urodeles (see Fig. 88, p. 157) 



sitic); e.fj.t, external gills (paia- , , , . , .. e 



sitic) which i.av, fevdoped from and also potential organs oi movement-* 



the implanted pi.-n- of skin; op, as shown by the well- developed muscula- 

 " I ' enull " n - ture by which they can be nicked back- 



wards. In other words these organs 



and these alone among the organs of the. Vertebrata possess the 

 qualiiieatimis which have to be postulated for the evolutionary 

 forerunner of the Vertebrate limb. 



In vie\\ of such considerations as those just set forth the present 

 wrili-r brlievi-s the most plausible working hypothesis nf tin- e\ulu- 

 linnary origin o\ 1h<- limbs haxiii-- regard to our present-clay kiu>\\ 

 ledge to be tliai uhi'-h interprets them as modified external -ills, he- 

 Inji-jiii^r to visceral arches fart hri' back in t he sei'ies t ban iln.se forming 

 the branchial an-lics of exist in-j \'- rtebrates, 'l'ln- limb ^inlle \\oul<l 



n ibi.- by pittheSlS, as on that of ( li-^-uliaur, be Interpreted as repre- 



senting a branchial arch sk-leton. ib<- iirierenee iVuni ih- Cc^enbauj- 



having to do r,M her with the nat inv of the j.rnjret in^ ]ji,,|, iiself. 



