489 



The Eleventh Day Stage. 



At about the middle of the eleventh day of development the 

 lungs and air-sacs show the effect of more rapid growth. The air- 

 sacs, as shown in Fig. 6, are relatively much larger than in prece- 

 ding stages. 



The prebronchial sac (pr.) now begins to expand at its distal end. 



The mesial moiety (S. 1) of the sub-bronchial sac is divided at 

 its extremity into two lobe-like bronches. The more mesial of these 



Fig. 6. Ventral view of uninjected luugs of ten and one-half day embryo. 

 Shows the air-sacs and their recurrent bronchi {r.br.). The recurrent bronchi of the 

 abdominal sac {ab.) and of the posterior intermediate sac (p.i.) are shown four only 

 part of their length, a.i., anterior intermediate air-sac; br., bronchi; oe., oesophagus; 

 pr., prebronchial air-sac; S.l, mesial moiety of subbronchial air-sac; S.2, lateral 

 moiety of sub-bronchial air-sac; tr., trachea. Outlined with camera lucida. 



branches extends toward the mid-ventral line between the two lungs 

 and nearly comes into contact with the corresponding branch from 

 the opposite lung (Fig. 6). The more laterally extending branch passes 

 ventral to the extra-pulmonary bronchus, which it partly engirdles. 



