330 THE DEVELOPMENT OF THE CHICK 
lungs of the adult ‘“‘have no peritoneal covering,” although this 
is denied by other authors. 
The air-sacs are terminal expansions of entobronchi or of the 
mesobronchus (Fig. 191). Some details of their later history 
may be noted as follows: The 
abdominal air-sacs are, accord- 
ing to all authors, the first to 
appear. It seems to me, how- 
ever, doubtful that the slight 
terminal expansion at the hin- 
der end of the mesobronchus 
should be regarded at its first 
appearance as the primordium 
of the posterior air-sac. How- 
ever this may be, they do not 
undergo any considerable ex- 
pansion until after the eighth 
day (cf. Fig. 191). Then they 
push through the hinder end of 
the pleuroperitoneal membrane, 
now fused with the lateral body- 
Fic. 191. — Lungs and air-sacs of a 
chick embryo of about 10 days. wall, and penetrate the latter 
(After Selenka.) just beneath the peritoneum. 
1, Cervical air-sac. 2 and 2’, inter- 
clavicular air-saec. 3, Anterior thoracic : ; 
air-sac. 4, Posterior thoracic air-sac. to expand into the abdominal 
5, Abdominal air-sac. ‘avity just behind the liver, 
thus evaginating the peritoneum. The enlarged sac is connected 
by a narrow tube with the hind end of the mesobronchus. The 
left sac is somewhat larger than the right. The expansion goes 
on rapidly and by the thirteenth to the fifteenth day they have 
reached the hinder end of the body cavity, and have already ex- 
About the tenth day they begin 
panded into it so far as to form fusions with the mesentery. 
According to Bertelli, the rudiments of the cervical sacs appear 
on the fifth day, but I doubt that they are distinguishable from 
the first entobronehus so early. They push forward first into 
the pleural cavity, afterwards entering the mediastinal tissue 
and so reach the neck. The interclavicular sac, which is single in 
the adult, arises on the sixth day as a pair of evaginations of the 
first entobronchus (according to Bertelli from the cervical sacs); 
they undergo fusion secondarily. 
