DEVKLOPMENT OF THE LYMPHATICS OF THE UXflS IX THE EMBRYO PIG. 59 



phatics, so that with each bronchus there are several veins and several sheets of 

 lymphatics developing. Thus the series of cone-shaped or pj'ramid-shaped lobules 

 are surrounded by plexuses of lymphatics. Along these plexuses the differentiation 

 of the connective-tissue layers takes place, for, when the lymjihatics invade these 

 areas, there is only an undifferentiated tissue, which is characteristic of the lung. 

 Flint suggested that the lymphatics followed the bronchi for a certain distance and 

 then turned away midway between them, because of some relative difference in the 

 density of the tissues. It is quite impossible to observe the relation to the veins in 

 uninjected sections, and consequently this point was not discussed in relation to 

 the problem of the (juestion of tissue densit3\ Notwithstanding this phase of the 

 development which Flint was unable to follow, there still remains considerable 

 probability in his suggestion. The fundamental reason for the direction of growth 

 is as yet entirely a mystery, but there seems to be little doubt that the principal 

 lines of lymphatic development are along the larger blood-channels; and, in general, 

 the veins are chosen, though the left duct may be considered as following the aorta. 



The much slower-growing Ij^mph- vessels on the bronchi follow each branch out 

 towards the periphery. The primary bronchus is surrounded by a very close-meshed 

 plexus, which consists of a large number of vessels; in cross-section one can count 

 from 50 to 75. However, this number is very greatlj' reduced on the secondarj* 

 bronchi, each of which has four or five trunks following it. These are closely bound 

 together by anastomosing collaterals. 



With reference to the secondary bronchi, almost the same series of events occur 

 as given above for the primary ones. These secondary bronchi are likewise marked 

 off by interlobular septa in which the lymphatics develop more rapidly than along 

 the bronchus whose lobule they mark off. The lymphatics around the bronchus 

 give off small vessels near each branch of the bronchus, and these pass across to 

 join the plexuses that surround the area of the lobule (figure 1, j^late 3). As the 

 new-formed bronchi grow larger they are, in turn, followed by two or three lymphat- 

 ics, which end, as did those around the secondary bronchi, by passing over to join 

 the septa or, if close to the pleura, the vessels there. These lymphatics that pass 

 from the bronchial sj'stem to join those in the septa follow the branches of the veins 

 which bend in from the septa to reach the capillary bed of the arterial tree. These 

 persist in the adult ,as the vessels that pass from the bronchus to the vein and thence 

 to the pleura (figure 2, plate 1). 



We will consider now the lymphatics that grow up from the retroperitoneal sac 

 into the caudal pole of the lower lobe. 



In 1906 F. T. Lewis described, in rabbit embiyos, a lymphatic sac just median 

 to the mesonephritic vein. Baetjer (1908) showed that it arises from the ventral 

 surface of the large vein which connects the two Wolffian bodies (embrj^os 17 to 

 23 mm.) ; Heuer, following Baetjer, found that num(>rous lymi)hatic sprouts arise 

 from this sac and invade the intestine through the mesentery. This sac sui)plies 

 lymph-vessels to the stomach, the liver capsule, the ■\\'olffian bodies, and the repro- 

 ductive glands. 



The lower pole of the lower lobe of the lung is continuous with the mesentery 

 in the early stages. As the embryo develops, this connection becomes a thin band 



