Florence E. Sabin 



379 



In general, when a vein buds off from another vein, it grows out at 

 nearly a right angle, while when a lymphatic buds from a vein, it grows 

 at the smallest possible angle, or, in other words, it grows parallel to 

 the vein. This difference in the direction of the growth of the lym- 

 phatic bud from the vein makes the valve at its orifice. (Fig. 4.) In 

 the paper of Dr. W. G. ]\IacCallum referred to above, he emphasizes 

 the continuity of the endothelium in the lymphatic system and the 

 fact that each endothelial cell comes from a preceding endothelial cell. 

 This idea is here fully confirmed and carried a step farther, namely, 

 that the endothelium of the lymphatic system buds off from the endo- 



ALH 



•ALH 



Fig. 5. 



Fig. 6. 



Fig. 5. Diagram of the lymphatic system iu an embryo pig 14.5 cm. long. x 3. 

 Acv, anterior cardinal vein; alh, anterior lymph heart ; pcv, posterior cardinal vein ; 

 Wb, Wolffian body. 



Fig. 6. Diagram of the lymphatic system in an embryo pig 15 mm. long. x 8. 

 Alh, anterior lymph heart, fv, femoral vein; k, kidney; sv, sciatic vein. 



thelium of the veins. In the stages considered in this paper none ol 

 the ducts nor sacs of the lymphatic system have any wall except a single 

 layer of endothelial cells. 



The proof that the h'lnphatic ducts bud off from the veins is as 

 follows: It has been established that the ducts invade the skin from 

 four points, two anterior and iwo posterior, and that the growth is 

 from centre to periphery. Starting from the time when the ducts 

 have completely covered the skin, every stage has been followed back- 

 ward until the ducts are extremely small and extend only a short dis- 

 tance from the vein. In this stage the opening into the vein is just 

 as perfect as in the later stages. Moreover, previous to the stage in 

 which this bud connected with the vein is found, there is no trace of 



