840 REPORT—1890. 
epiblast cells. It is in connection with these variations that controversy has arisen 
as to the primitive mode of development of the gastrula, a pomt to which I shall 
return later on. 
Among the higher Metazoa or Ccelomata the extraordinary modifications in the 
position and in every conceivable detail of formation of the 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 explanation, 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 hvpoblast, of Ccelenterata. 
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! have given us evidence in the extraordinary 
modifications presented by the embryonic and larval respiratory organs of 
Amphibians. 
onfining 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 Ichthy- 
ophis 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; and in Rana opisthodon, according to Boulenger, larval respira- 
tion 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 larvee 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 view 
contradicted by the close agreement in their relations in the two groups, as well as 
by the absence of any real break between external and internal gills in Amphibians. 
It is well known that the frog and the newt differ greatly in important points 
of their development. The two-layered condition of the epiblast in the frog is a 
marked point of difference, which involves further changes in the mode of formation 
of the nervous system and sense organs. The kidneys and their ducts differ 
considerably in their development in the two forms, as do also the bloodvessels. 
Concerning the early development of the bloodvessels, there are considerable 
differences even between allied species of frogs. In Rana esculenta Maurer finds 
shat there is at first in each branchial arch a single vessel or aortic arch, running 
directly from the heart to the aorta: from the cardiac end of this aortic arch a 
vessel crows out into the gill as the afferent branchial vessel, the original aortic arch 
losing its connection with the heart, and becoming the efferent branchial vessel. 
Afferent and efferent branchial vessels become connected by capillaries in the gill, 
and the course of the circulation, so long as gill-breathing is maintained, is from the 
heart through the truncus arteriosus to the afferent branchial vessel, then through the 
gill capillaries to the efferent branchial vessel, and then on to the aorta. When the 
pulmonary circulation is thoroughly established the branchial circulation is cut off 
by the etferent vessel reacquiring its connection with the heart, when the blood 
naturally takes the direct passage along it to the aorta, and so escapes the 
gill capillaries. 
In Rana temporaria the mode of development is very different: the afferent and 
1 Pp. and F. Sarasin, Ergebnisse naturnissenschaftlicher Forschungen auf Ceylon, 
vol, ii. chap, i. pp, 24-38. 
