152 



SCIENCE 



[N. S. Vol. XLIV. No. 1127 



there are vessels which come up behind the 

 diaphragm. The ducts which grow down 

 from the neck meet in a plexus which sur- 

 rounds the trachea. In the primitive lung, 

 the general pattern of the organ is simple; 

 it is obviously blocked off into large lobules 

 by wide connective tissue septa. In the 

 center of each lobule are the bronchus and 

 the artery, in the septa are the veins. At 

 the hilum the tracheal lymphatics divide 

 into three plexuses, one spreading on to the 

 pleura, a second following the bronchi and 

 arteries, and the third the veins. The 

 plexus which follows the veins grows 

 rapidly to the pleura and spreads around 

 the border of each primitive lobule, blocking 

 off the pleura into polygonal areas. From 

 this pattern the pleural lymphatics develop. 

 The pleura is blocked off into its polygonal 

 areas by the lymphatics when the embryo is 

 about 5 cm. in length. At a much later 

 stage, when the bronchi begin to develop 

 atria and air sacs at their tips, the lymphat- 

 ics grow down the center of the lobule along 

 the bronchi. Just where the atria begin, 

 the lymphatics turn sharply from the 

 bronchi and pass out to the septa, so that 

 the walls of the air sacs are without lym- 

 phatics. 



The lymphatics of the diaphragmatic sur- 

 face of the pleura grow up behind the dia- 

 phragm from the retroperitoneal sac, and 

 injections of this surface of the lung in 

 later stages fill up the pre-aortic, abdominal 

 lymph glands. This relation of the pleural 

 lymphatics to the abdominal lymphatics I 

 believe to be of importance. 



The development of the ducts to the intes- 

 tines, and their differentiation within the 

 intestinal wall into the ultimate lacteals of 

 the villi, have also been worked out. The 

 method of injection in the embryo affords 

 an excellent opportunity to test the present 

 belief in the partial invasion of organs by 

 lymphatic vessels. For example, lymphatics 



have not been demonstrated in the adult 

 liver beyond the capsule and the connective 

 tissue septa, nor in the spleen beyond the 

 capsule. It is well known that lymphatics 

 are abundant in tendons ; but they have not 

 been demonstrated in striated muscle. On 

 the other hand, it has been definitely shown, 

 both in the embryo and in the adult, that 

 there are no lymphatics in the central nerv- 

 ous system. 



To this very general account of the lym- 

 phatic system in the mammal certain inter- 

 esting facts from comparative anatomy 

 must be added. It has long been known 

 that there are pulsating lymph hearts in 

 the amphibia. These lymph hearts arise as 

 lymph sacs from the vertebral veins in the 

 neck and from the coccygeal veins at the 

 root of the tail. These sacs are close to the 

 myotomes and develop striated muscle in 

 their walls. In the birds there is a very 

 interesting lymphatic system. There is a 

 jugular lymphatic plexus which later be- 

 comes a lymphatic gland, and a caudal pul- 

 sating lymph heart, which develops from 

 the coccygeal veins. In mammals the lymph 

 sacs develop into groups of lymph glands, 

 which may be called the primary glands for 

 each region, while secondary glands de- 

 velop along the lymphatic duets. 



In this brief resume of the lymphatic 

 system I have given only facts which can 

 be clearly demonstrated. There are these 

 sacs against the veins, and if injections are 

 made from them one can demonstrate a 

 gradually increasing plexus of vessels. 

 These facts, however, but lead us on to seek 

 their meaning. What are lymphatic capil- 

 laries, how do they arise, and how do they 

 grow ? There is general agreement that the 

 lymphatics arise from certain centers and 

 grow toward the periphery; but there are 

 two theories as to how they grow. The 

 theory which I bold is that the lymphatics 

 arise from the endothelium of the veins and 



