DEVELOPiMENT OF THE LYMPHATICS OF THE LUNGS IX THE EMBRYO PIG. 51 



(3) Finally, the vessels of the lung are best injected by a puncture just ventral 

 to the trachea (the tracheal plexus) and behind the arch of the aorta. Here the 

 tracheal plexus is always extravasated, but the lung-vessels fill up nicely. 



The embryos older than these mentioned, that is, longer than 7 cm. (or after 

 the valves are formed), are much more difficult to inject, and this difficulty increases 

 with further development. The method employed has been to inject directly into 

 the connective-tissue septa of the lung and to continue the injection slowly until 

 there is some extravasation at the point of puncture, when a part of the lung sur- 

 rounding the area of extravasation is well injected. This method has been very 

 satisfactory in all specimens that were obtained very soon after the removal of the 

 uterus; most of the injections were made while the heart was still beating. 



In order to study the relations between the blood-vessels, bronchi, and lym- 

 phatics, multiple injections had to be made. Various combinations were employed. 

 In some, the lymphatics were injected together with veins and arteries; in others 

 with either veins or arteries alone. Again, the lymphatics and the bronchi were 

 injected; and in still others the lymphatics were combined with either veins or 

 arteries. In these multiple injections prussian blue, India ink, and carmine were 

 used, the lymphatics being injected with either the blue or the ink. 



The specimens in which three s,ystems were injected were difficult to clear, 

 unless only the large bronchi and blood-vessels were filled. 



In order to trace the vessels more accurately, many of the injected lungs were 

 embedded in paraffin and cut in thick serial sections (100 to 500 ^t); these were 

 mounted in balsam but not stained. Other lungs were cut at 10 to 20 ju and stained 

 similarly to the series already referred to. 



All measurements of embryos refer to crown-rump diameter and were taken 

 before fixation, as is customary in this laboratory. The illustrations are labeled 

 "C. R. — "; this refers to the crown-rump measurement. 



In 1906, Flint published his study on the development of the lungs in the pig, 

 and his work has been taken as a basis of the general structure of the lungs, especially 

 with reference to the development of the bronchi and blood-vessels. He reviewed 

 all the important literature on the embryology of the mammalian lung, studied the 

 lymphatics in sections, and briefly summarized their structure and distribution at 

 various stages, but he did not attempt to inject them. I have been able to confirm 

 most of his observations. However, he labored under the difficulty of having 

 neither reconstructions nor injections. He gives a short summary of each stage, 

 and of these summaries I quote the more important parts : 



Stage 3 cm.: At the root of the lung a few dilated lymphatics may l)c noted near the bronchi 

 and pulmonary vessels; however, they have not grown beyond this point into the substance of the 

 lung wings. 



Stage 5 cm.: From the root of the lung the lymphatics have gone some distance into its sub- 

 stance. They have thin walls composed of young fibrils lined with endothelium with occasional 

 valves. They are confined, however, to the inunediate neighborhood of the main bronchi and their 

 chief subdivisions. 



Stage 7 cm.: The most interesting change, however, lies in the further growth of (he jyiiiphatics, 

 which in the earlier stages are found in the root of the lung in the neighborhood of the pulmonary 

 ves.sels and the large bronchi. As they grow in, they accompany these structures for a distance; 



