THE EMBRYOLOGY OF THE BIRd's LUNG 453 



chloroform might evaporate. The evaporation of the chloro- 

 form served to draw the cedar oil from the lumina of the various 

 branches of the bronchial tree into the Imig tissue and to fill 

 the spaces thus made with air. When this preparation was re- 

 placed in pure cedar oil, the difference between the refractive 

 index of the imprisoned air and the surrounding medium gave 

 the lung tubes the appearance of being filled with a metallic cast. 

 Thus the minute air passages that could not be injected by other 

 means were made clear. The finer details would disappear af- 

 ter a few minutes as the cedar oil percolated into them, but the 

 same specimen, if carefully manipulated can be treated repeat- 

 edly without apparent injury, and a complete picture could fin- 

 ally be obtained. This method was successfully used in tracing 

 the development of the bronchial tree up to the eighteenth day 

 of incubation. 



For later stages, celloidin and Wood's metal injections af- 

 forded the most helpful preparations. Such injections were also 

 attempted of earlier stages, but the uncertainty of successful 

 preparations and the destruction of the specimen employed 

 made the air injections more satisfactory. This was especially 

 true since the air injections showed fine points of detail that were 

 not revealed by the more limited penetration of the fluid cel- 

 loidin and the heated Wood's metal. Several preparations of the 

 adult lung were made with the Wood's metal injections. Pre- 

 ceding the Wood's metal casts, the lungs of the freshly killed 

 fowl were distended under pressure with 80 per cent alcohol 

 until the air-sacs were fully expanded, after which the entire 

 bird was immersed in alcohol for twenty-four hours or more be- 

 fore attempting metallic injection. 



For histological study of the air capillaries of the adult bird, 

 the inilmonary apparatus was injected under pressure with cor- 

 rosive acetic fluid and by this means the lungs were fixed in a 

 distended condition and the air capillaries were not collapsed. 



