DEVELOPMENT OF THE LYMPHATICS OF THE LUNGS IN THE EMBRYO PIG. 53 



The thoracic duct, as has been shown by Sabin (1913), Baetjer (1908), and 

 Kampmeier (1912), is complete — that is, it connects the jugular sac with the retro- 

 Ijeritoneal sac — in a pig embryo 2.5 cm. long. Very soon after this the first evidence 

 of the i^ulmonary supply may be found. I have obtained partial injections at 

 2.8 cm., and have found some small vessels in serial sections at 2.6 cm.; so it is 

 evident that these sprouts are either formed from the thoracic duct as it grows down 

 or very soon after the primary system is completed. 



About midway between the jugular anastomosis and the arch of the aorta the 

 thoracic duct leaves its position lateral to the trachea and bends dorsalward to lie 

 near the dorso-lateral border of the esophagus. In this ]:)osition it comes down 

 behind the arch of the aorta. This transition is shown by l^abin (1913, figures 12 

 and 13). Just at the point where the duct begins to bend dorsally the earliest sprout 

 to the lung is formed. At this point a single large vessel buds off from the thoracic 

 duct and passes down over the arch of the aorta to reach the hilum of the lung. 

 This vessel unites with the vessels that grow up from the thoracic duct just caudal 

 to the arch of the aorta and forms the lower part of the tracheal plexus. This vessel 

 usually persists in the adult as one of the drainage trunks from the hilac nodes to the 

 thoracic duct. It is shown in figure 5, plate 1, and figure 2, plate 4, marked with an 

 asterisk. From the region of the thoracic duct, where this vessel buds off to a point 

 about the level of the aortic arch, a number of other vessels are formed very soon 

 afterwards. These vessels arise very close together and grow across to the lateral 

 wall of the trachea, where they anastomose and form the primitive left tracheal 

 plexus; they lie in the undifferentiated mesenchymal tissue that surrounds the 

 tracheal lumen. These lymphatics have formed a plexus by the time the embryo 

 has reached a length of 3 cm. From this plexus vessels grow across the trachea 

 to anastomose with other vessels from the similar plexus on the opposite side; other 

 Ij'mphatics grow up the trachea and form a coarse-meshed plexus around it. This 

 is the anlage of the adult supply of that structure. But the most important of the 

 branches of this plexus, as far as the present work is concerned, arc those from the 

 lower part. These pass down the trachea and, being joined by other vessels that 

 leave the duct near the arch, pass up over the bifurcation and into the lung. The 

 left tracheal plexus is shown in figure 5, plate 1, and figures 1 and 3, plate 2. Here 

 must be noted the fact that the plexus of the left side supplies the greater portion 

 of the ventral surface of the trachea and forms the largest part of the great sheet of 

 lymphatics around the primary bronchi. Later these vessels anastomose freely 

 with those from the right side. It is important to call especial attention to the 

 difference in the richness of the supply of the dorsal and the ventral surfaces of the 

 trachea. There are vessels that grow to each from the left plexus, but a much 

 greater number pass to the ventral surface than to the dorsal. Thus the plexus 

 formed from the two lateral groups is much more closely meshed on the ventral 

 surface, and from it is derived the greater part of the lung supiily. Over the bifur- 

 cation there is a very complex group of vessels, and these form tubes around the 

 principal bronchi as they grow on into the lung. 



Below the level of the hilum several vessels, three or four in number, grow up 

 from the thoracic duct and its plexus surrounding the aorta, to join with the large 



