150 



SCIENCE 



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



during that year I was working on the de- 

 velopment of the lymphatic system. 10 I 

 began the investigation by injecting the 

 foot-pads of young pig embryos. This pro- 

 cedure never fails to demonstrate lymphat- 

 ics in the adult, and the same is true of fetal 

 stages, but it was soon found that in em- 

 bryos less than 3 cm. in length it was neces- 

 sary to introduce the needle nearer the cen- 

 tral veins in order to find lymphatics. By 

 a long series of such injections the fact was 

 gradually established that the skin of the 

 embryo is invaded by lymphatics from two 

 general regions — the neck and the groin. 

 By noting the lines of growth of these in- 

 vading vessels it was possible to obtain 

 injections, showing the extent'of the inva- 

 sion of the skin for each stage. Moreover, 

 in making these injections into the translu- 

 cent skin of the embryo it became evident 

 that in order to fill the lymphatics the 

 needle must be introduced at a very exact 

 level. When the needle cuts the lymphatics, 

 the vessels can be seen to fill up from the 

 oblique opening of the needle, without any 

 extravasation if the pressure is light. If 

 the needle is entered too superficially a bleb 

 is always formed: if too deeply, the injec- 

 tion mass spreads out in straight lines, very 

 characteristic and very different from lym- 

 phatics. These observations emphasize the 

 lymphatic capillary as a definite vessel 

 located at a specific level. Through a long 

 series of such injections these definite lym- 

 phatic vessels were traced back to tiny buds 

 close to the veins. The theory was then 

 advanced that the entire lymphatic system 

 consists of definite vessels of endothelium, 

 which grow as blind buds from the endo- 

 thelium of the veins and partially invade 

 the body. The theory throws the emphasis 

 on endothelium as the essential tissue of the 



io Sabin, F. E., Johns Hopkins Hospital Ee- 

 ports, Monographs, New Series, No. 5, 1913. Gives 

 a list of the literature. 



lymphatic system, and premises that the 

 endothelium of the lymphatic system is de- 

 rived from the endothelium of the veins. 

 This means that lymphatic vessels arise 

 as an active growth of endothelial cells and 

 are not formed by a passive dilatation of 

 spaces. The outgrowth theory has not been 

 established without opposition. There has 

 been, indeed, a vigorous effort in this coun- 

 try to re-establish the older hypothesis of 

 the origin of lymphatics from tissue-spaces, 

 but in my judgment these efforts have not 

 been successful. 



I shall now outline briefly certain facts 

 which have been established concerning the 

 development of the lymphatic system. The 

 lymphatic system begins in the human em- 

 bryo of about 10 mm. in length — that is, 

 during the sixth week of development. The 

 first lymphatics are blunt buds which come 

 from the internal jugular veins at the root 

 of the neck. They are filled with blood 

 which backs into them from the vein. These 

 buds soon establish connections with, each 

 other and form a plexus which develops 

 into a large sac, having its base on the in- 

 ternal jugular vein and arching into the 

 posterior triangle of the neck. From this 

 sac, which is astonishingly large, lymphat- 

 ics grow out to the skin of the head and 

 neck, to the thorax and arm, and partially 

 invade the deep structures of the head. 

 From the portion of the sac in the posterior 

 triangle of the neck, vessels grow forward 

 and form an extensive plexus along the ex- 

 ternal jugular vein. The knowledge of the 

 form of this sac, of its position with refer- 

 ence to the internal jugular vein, and the 

 pattern of the plexuses which develop from 

 it, has unraveled the complicated and 

 puzzling relations of the lymphatic ducts to 

 the chains of lymph glands in the neck. 

 The sac itself is transformed into different 

 groups of lymph glands which might be 

 analyzed as the primary lymph glands of 



