54 DEVELOPMENT OF THE LYMPHATICS OF THE LUX(!S IN THE EMBRYO PIG. 



vessel which has been described as the first to the hing and which comes over the 

 arch to reach the hiluni. These vessels from the duct below the hilum form a 

 l)lexus with the vessel from above, as has been described. It is well known that 

 the thoracic duct is double below the level of the arch of the aorta and that the two 

 divisions are connected by numerous anastomostic vessels (figure 1, plate 2). This 

 system is the anlajje of the vessels that surround the aorta in the adult. This 

 relation has been figured by Ileuer (1909). One of the lymphatics that pass up 

 from below to join the first vessel from the thoracic duct above leaves the duct near 

 the diaphragm and is consccjuently very conspicuous in injections of this region. 

 Heuer has figured this lymiihatic as one that goes to the heart, a conclusion entirely 

 justifiable from the general appearance of the inject(>d specimen. Figure 1, plate 2, 

 is from a dissected embr3'o 4 cm. long, in which the lymphatics were injected from 

 the retroperitoneal sac. The thoracic duct and part of the left tracheal plexus are 

 injected, and the extension of the plexus down on the bronchus is also shown. 

 Below the arch may l)e seen some of the vessels that grow up to meet the branch 

 from al)o^•e. These vessels have been cut off, with the arch, to expose the tracheal 

 plexus. The double duct is also shown, the more ventral element being the one 

 figured by Heuer. 



The pulmonary vessels reach the hilum when the embryo is about 2.8 cm. long, 

 and can be seen in sections at 3 cm. (see figure 1, plate 1). The lung-tissue is at this 

 time very slightly difTerentiated mesenchyme, containing the earlj' bronchi and 

 blood-vessels. For a further description of the structure of the lung at this stage 

 see Flint (1906). These early lymphatics are grouped in an irregular manner in 

 the hilum of the lung and may be found at 2.9 and 3 cm. in sections. But I have 

 not been able to inject them earlier than 3.3 and 3.5 cm. Figure 1, plate 1, is of a 

 section from an embryo 3 cm. long, in which the blood-vessels were injected wliilc 

 the embryo was still living. The lymphatics are shown as a few dilated spaces 

 (blue) in the hilum. These vessels are beginning their invasion of the lung-tissue 

 while the tracheal plexus is forming. It is necessary, however, to comi^lete the 

 description of this plexus before considering the portion of this study which relates 

 to the lung proper. The development of the vessels within tlie lung-substance will 

 be considered after the formation of the right lymphatic plexus has been described. 

 It is important, however, to note here that all the vessels to the left lung come from 

 the closely united group of vessels on the trachea and around the aortic arch, as has 

 Ix-eii described. This will be studied in relation to the first vessels to the lung on 

 the right side, which will next be considered. 



On the right side the development is, in general, similar to that on tlie left, but 

 differs in a few particulars, chiefiy relating to and in consecjuence of the asymmetry 

 of the vascular system. The right duct is primarily to the heart, or perhaps to the 

 vena cava, since it follows that vessel to reach the cardiac base. But while the heart 

 supply is at first only from the right side, the vessels to the lung and the trachea 

 develop at about the same time. The right duct grows caudalward parallel to the 

 thoracic duct to the point where the vena cava arches ventralward to reach the 

 heart. There it divides, and one branch follows the posterior wall of the vena cava 

 to reach the cardiac base, while the other passes into the hilum of the lung. The 



