of the Paired Limbs of Vertebrates. 233 



as to my mind seem not to be adequately explicable except on 

 this assumption. 



In the Urodela the External Gills 1 are well-known structures — 

 serially arranged projections from the body-wall near the upper 

 ends of certain of the branchial arches. When one considers the 

 ontogenetic development of these organs, from knob-like out- 

 growth from the outer face of the branchial arch, covered with 

 ectoderm and possessing a mesoblastic core, and which frequently 

 if not always appear before the branchial clefts are open, one 

 cannot but conclude that they are morphologically projections of 

 the outer skin and that they have nothing whatever to do with 

 the gill pouches of the gut wall. Amongst the Urodeles one such 

 gill projects from each of the first three branchial arches. In 

 Lepidosiren there is one on each of Branchial Arches I — IV-. In 

 Polypterus and Galamoichthys there is one on the Hyoid arch. 

 Finally in many Urodelan larvae we have present at the same 

 time as the external gills a pair of curious structures called 

 balancers. At an early stage of my work on Lepidosiren while 

 looking over other Vertebrate embryos and larvse for purposes 

 of comparison, my attention was arrested by these structures, and 

 further examination by section and otherwise convinced me that 

 they were serial homologues of the external gills, situated on the 

 Mandibular arch. On then looking up the literature I found 

 that I was by no means first in this view. Rusconi had long ago 

 noticed the resemblance and in more recent times both Orr and 

 Maurer had been led to the same conclusion as I had been. 

 Three different observers having been independently led to 

 exactly the same conclusions, we may I think fairly enough 

 regard the view I have mentioned of the morphological nature of 

 the balancers as probably a correct one. 



Here then we have a series of homologous structures projecting 

 from each of the series of visceral arches. They crop up in the 

 Crossopterygii, the Dipnoi, and the Urodela, i.e., in three of the 

 most archaic of the groups of Gnathostomata. But we may put 

 it in another way. The groups in which they do not occur are 

 those whose young possess a very large yolk sac (or which are 

 admittedly derived from such forms). Now wherever we have a 

 large yolk sac we have developed on its surface a rich network of 

 blood vessels for puposes of nutrition. But such a network must 



1 For a comprehensive survey of the External Gills of Vertebrates v. Clemens, 

 Anat. Hefte, Abt. 1, Bd. v. 



2 In Lepidosiren as in Protnpterus the original relations of the external gills 

 become somewhat obscured during Ontogeny by their becoming raised upon a 

 common stalk on each side, the base of attachment of this coming by differential 

 growth to lie in the region of the pectoral girdle. This relation of external gills 

 to pectoral girdles is of course quite secondary, as is indicated by their blood-supply 

 as well as by their development. 



