1 66 MILLER. [Vol. VIII. 



3. Digestion in pepsin after the air-passages and blood- 

 vessels had been injected with colored celloidin. 



4. Reconstruction as well as free-hand modeling. 



The first method is of greatest service in the study of the 

 lungs of Amphibia and Reptilia. In these animals the large 

 air-sacs permit a very careful study of the interior of the lung 

 after it has been blown up, dried and cut open. The mam- 

 malian lung, however, is hard to dry, and the best specimens 

 are of little value in the study of the finer structure. 



Injections with wax and with Wood's metal proved of great 

 value in the study of the blood-vessels, but are of little value 

 in the study of the air-passages within the lobule. It is of 

 great service in studying the bronchial tree of embryos which 

 are sufficiently advanced to permit a cannula being tied in the 

 trachea. But all that can be obtained with Wood's metal or 

 with wax can be obtained equally well or better with celloidin. 

 It is by all odds the most desirable method. The celloidin is 

 to be colored with vermilion, Prussian blue or chrome yellow. 

 After the vessels have been injected the organ may be placed 

 in water for some time, and then the tissues are digested with 

 pepsin and one half per cent, hydrochloric acid. For this 

 latter procedure I am under obligations to Dr. Mixter of 

 Boston. The digestion, of course, takes place best in the 

 thermostat ; and when the tissues are thoroughly softened, 

 which often takes days, they are to be washed off in flowing 

 water. In all cases I found that the corrosions are best pre- 

 served in a mixture of equal parts of glycerine, alcohol and 

 water. 



Direct modeling I found of great service in the study of 

 small corrosions of the tips of the bronchi of embryos. For 

 the more complex lungs the reconstruction method of Born 

 was at last resorted to, by which, after devoting nearly two 

 years to a single specimen, the desired light was obtained. 

 The lung was first prepared by making a triple injection, by 

 which the capillaries were filled with Prussian blue, the 

 arteries with vermilion, and the veins with ultramarine-blue 

 gelatine. A single lobule was then removed, imbedded in 

 paraffine, and cut into sections 20 /u, thick. The magnifying 



