26 PROFESSOR A. M. MARSHALL. 



mesoblast in different and often in closely allied forms have given rise 

 to ardent discussion, and have led to the proposal of theory after 

 theory, each rejected in turn as only affording a partial explana- 

 tion, and now culminating in Kleinenberg's protest against the use of 

 the term mesoblast at all, at any rate in a sense implying any possibility 

 of comparison with the primary layers, epiblast, and hypoblast, of 

 Coelenterata. 



This is not the place to attempt to decide so difficult and technical 

 a point, even were I capable of so doing, but we may well take warning 

 from this extraordinary diversity of development, the full extent of 

 which I believe we as yet realise most imperfectly, that in our attempts 

 to reconstruct ancestral history from ontogenetic development we have 

 taken in hand no light task. To reconstruct Latin from modern 

 European languages would in comparison be but child's play. 



Of the readiness with which special developmental characters are 

 acquired by allied animals the brothers Sarasin 1 have given us evidence 

 in the extraordinary modifications presented by the embryonic and 

 larval respiratory organs of Amphibians. 



Confining ourselves to those forms which do not lay their eggs in 

 water, and in which consequently development takes place within the 

 egg, we find that Ichthyophis and Salamandra have three pairs of 

 specially modified external gills. Nototrema has two pairs; Alytes 

 and Typhlonectes have only a single pair, which in the latter genus 

 take the form of enormous leaf-like outgrowths from the sides of the 

 neck. In Hylodes and Pipa there are no gills, the tail acting as 

 the larval respiratory organ ; in Rana opisthodon, according to 

 Boulenger, larval respiration is effected by nine pairs of folds of the 

 skin of the ventral surface of the body. 



Most of these extraordinarily diversified organs are clearly secondarily 

 acquired structures ; it is possible that they all are, and that external 

 gills, as was suggested by Balfour for Elasmobranchs, are to be 

 regarded as embryonal respiratory organs acquired by the larvae and of 

 no ancestral value. The point, however, cannot be considered settled, 

 for on this view the external gills of Elasmobranchs and Amphibians 

 would be independently acquired and not homologous structures, a 



1 P. and F. Sarasin, Ergebnisse naturwissenschaftlicher Forschungen auf 

 Ceylon, vol. ii. chap. i. pp. 24-38. 



