. Text-book of embryology. Embryology. 450 EMBEYOLOGY OF THE LOWEE VEETEBEATES 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 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 gro
. Text-book of embryology. Embryology. 450 EMBEYOLOGY OF THE LOWEE VEETEBEATES 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 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 hypo
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