450 EMBEYOLOGY OF THE LOWEE VEETEBEATES ch. 



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- 

 ing VertelDrates. It is therefore concluded 

 that in an earlier phase of its evolution 

 the phylum whose modern representatives 

 we call Vertebrates was characterized by 

 the possession of a series of external gills 

 extending tailwards beyond the limit 

 reached by the branchial region of exist- 

 ing Vertebrates. 



But such external gills are potential 

 organs of support — as shown by the 

 "balancers" of Urodeles (see Fig. 88, p. 157) 

 — and also potential organs of movement — 

 as shown by the well -developed muscula- 

 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 

 qualifications winch have to be postulated for the evolutionary 

 forerunner of the Vertebrate limb. 



In view of such considerations as those just set forth the present 

 writer believes the most plausible working hypothesis of the evolu- 

 tionary origin of the limbs— having regard to our present-day know- 

 ledge—to be that which interprets them as modified external gills, be- 

 longing to visceral arches farther back in the series than those forming 

 the branchial arches of existing Vertebrates. The limb girdle would 

 on this hypothesis, as on that of Gegenbaur, be interpreted as repre- 

 senting a branchial arch skeleton, the difference from the Gegenbaur - 

 view having to do rather with the nature of the projecting limb itself. 



Fig. 204. — Left side of head 

 region of a larva of Ra/na 

 temporaria upon which, four 

 days previously, a piece of 

 skin from the branchial region 

 of another embryo had been 

 grafted. (After Ekmau, 1913.) 



e.f/.I, II, III, external gills (auto- 

 sitic) ; e.g.t, external gills (para- 

 sitic) which have developed from 

 the implanted piece of skin ; op, 

 operculum. 



