THE EMBRYOLOGY OF THE BIRd's LUNG 43 



chial circuits. Conduits de\'eloped from tliem turn back into the 

 lung and join the network of air passages, so that, the air, httle 

 changed, and warmed to body temperature, is carried back into 

 the hmg for aeration of the blood. The lung, although relatively 

 small, is highl}^ vascular and a very efficient organ of respiration. 

 Rapid respiratory changes are favored by the structures de- 

 scribed and by the very intimate relations between air capillaries 

 and blood capillaries. Taken together they constitute a felt- 

 work of vascular and air capillaries mingled together. 



It is clear from Campana's text that he noticed the recurrent 

 bronchi of the adult on the four air-sacs from which they have 

 their origin. He makes comment upon them in each case. His 

 reference to those of the abdominal sac shows that he thought of 

 them as combinations of parabronchi. On page 54 of his memoir 

 he says: "On voit a la face dorsale du poumon des grosses ter- 

 tiaries, on pourrait presque dire des secondaires reconstituees 

 par la reunion des tertiaries plus fines, aboutir sur les parois du 

 septieme infundibulum (fig. 13, C) ou, ce qui revient au meme, 

 s'ouvrir dans la termination des dernieres secondaires externes." 



On the whole, Campana's observations afford a sort of pro- 

 phetic anticipation of the full recognition of recurrent bronchi. 



The discovery of the recurrent bronchi brings a new point of 

 interest into the study of the lungs of Sauropsida, and it is much 

 to be desired that extensive comparative studies may be entered 

 into that will embrace a careful consideration of the air-sacs and 

 their relations in reptiles. - 



2 The variations in chronology of chick embryos represented in tlie illustra- 

 tions of the standard references is so great that an additional comment is 

 appropriate. Comparison of the figures of Duval and of Keibel and Abraham 

 shows variations sometimes exceeding twenty-four hours (cf. Duval, fig. 142, 

 140 hrs.; and Keibel and Abraham, fig. 27, 114| hrs.). While it would be a satis- 

 faction to embryologists to have the chronology standardized, the essential point 

 is a correct analysis of anatomical conditions and the sequence of events. 



