458 MR..G. W. BUTLER ON THE SUBDIVISION OF [Nov. 19, 
rosis,’’ while the ‘‘intermediate”’ or ‘ diaphragmatic” air-sacs lie 
posterior to this in a layer of tissue which, at this stage, forms the 
thicker element of the avian diaphragm. 
At the beginning of the 12th day (cf. Plates XLVI., XLVII. 
figs. 14 and 15) we find that these air-sacs have increased in propor- 
tional size. Compare, for instance, the relative extent of the anterior 
diaphragmatic sacs (s.a’) in figs. 12 and 15, which are taken 
through an approximately corresponding region of embryos of the 
10th and 12th day respectively. 
As these air-sacs develop, they, so to speak, split the layer of the 
diaphragm in which they lie, part going, with the darkly staining 
layer and tissue in front, to form the ‘ pulmonary aponeurosis,”’ 
and part, that lies postero-ventrally to the air-sacs, forming the 
‘‘oblique septum.”? In the end the two air-sacs referred to come 
into contact, and any membranous diaphragmatic tissue that may 
lie between the apposed walls is quite insignificant. 
The avian diaphragm is thus seen to be completed as a single 
structure, and its separation into its two laminz is a secondary 
detail arising in connexion with the development of the two pairs of 
intermediate or diaphragmatic air-sacs, which first penetrate it at a 
time when no distinct line can be drawn between the tissue that 
goes to form it and that of the developing lung itself. 
III. (6). Certain previous Opinions with regard to the 
Avian Diaphragm. 
With regard to the question of the homology, if any, between the 
Mammalian and Avian diaphragms, Sappey, whose most excellent 
monograph on the respiratory apparatus of the adult bird (1) was 
published in 1847, expresses his opinion as follows (p. 21) :—‘‘ Dans 
les oiseaux le diaphragme comprend deux plans qui se confondent 
a leur point de départ, mais s’isolent bientét pour suivre l’un une 
direction transversal, l'autre une direction oblique.” ...... “Le 
premier a pour analogue dans homme et les mammiféres toute la 
partie de ce muscle qui s’ins¢re a la face interne des cétes ; le second 
représente les piliers du diaphragme ;”’ and he gives his reasons for 
regarding the attachment of the paired partly muscular fibrous tracts 
in the anterior dorsal region of the oblique septum as homologous 
with those of the pillars of the diaphragm to the lumbar vertebree. 
The presence or absence and the distribution of muscle-fibre seems 
to have considerable weight with him and others in dealing with 
questions of homology’; and with this the questions of nerve-distri- 
bution must be also considered. 
With reeard to this latter, Sappey says (1, pp. 25 and 26) that 
(what Huxle+ afterwards called) the “pulmonary aponeurosis” is 
supplied from: the intercostal nerves, and suggests that this mode of 
supply is similar to that by the phrenic nerve, both being by branches 
* The question is apt to present itself, however, is muscle-fibre of great 
importance in such a case? Might we not have a septum homologous to the 
mammalian diaphragm without any muscle developed in it at all, ‘simply con- 
sisting, so to speak, of the pleural and peritoneal membranes back to back ? 
