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, 



FIG. 191. Lungs and air-sacs of a now fused with the lateral body- 

 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- A u t f u f f u i f i K AO -i n 



clavicular air-sac. 3, Anterior thoracic About tne tentn da y tne y be g in 



air-sac. 4, Posterior thoracic air-sac, to expand into the abdominal 



5, Abdominal air-sac. cavity jugt behmd the Yly ^ 



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. x 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- 

 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 entobronchus 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. 



