The Origin and Development of the Lymphatic System. 45 



phatic sacs as of the more superficial ones; yet the inference that 

 they arise in the same manner is in harmony with our modern knowl- 

 edge of morphology. I wish to postpone a discussion of Kampmeier's 

 evidence until I have described the thoracic duct in a human embryo, 

 and discussed the development of the theories opposed to my own — of 

 which Kampmeier's work is a part (See. VIII). 



2. Primary Lymphatic System in Human Embryos. 



On this description of the lymphatic system in one mammalian Form 

 I shall base an account of what is known of the primary system in other 

 forms. All of the primary lymphatic saes have been found in the 

 human embryo. The jugular sac is the earliest to appear. It has 

 been found as a few blood-packed buds in two embryos measuring 8 

 and 9 mm. (Mall collection, No. 397 and No. 163, Sabin 134.) They 

 are on the lateral surface of the anterior cardinal vein near the duct of 

 Cuvier and were described as veins (Sabin 133, 131), since at that 

 time the blood-filled buds were thought to be blood capillaries. The 

 blood-packed buds extend along the lateral surface of the anterior car- 

 dinal vein and very early develop sprouts that project dorsalward. 

 These are plain in an embryo measuring 11 mm. (Mall collection. Mo. 

 353, figs. 7 and 8, Sabin 133, and figs. 490 and 491, Sabin 134.) The 

 figures of this specimen, together with the reconstruction of Lewis 

 (Harvard collection No. 1000, measuring 16 mm.; Xo. 189, measuring 

 11.5 mm.; and No. 1332, measuring 16 mm., figs. 1-3, Lewis 78), 

 show that the jugular lymphatics in the human embryo not only 

 bud from the anterior cardinal veins, but from the posterior cardinal 

 vein, and the common stem or plexus which forms the origin of the 

 primitive ulnar and thoracoepigastric veins. A largo extension of 

 the jugular sac along the primitive ulnar vein is characteristic of 

 human embryos (fig. 12, Sabin 133, or fig. 493, Sabin 134, and figs. 

 *. ; and 3 after Lewis 78). 



In a human embryo 10.5 mm. long I have found a small sac only 

 partly filled with blood (figs. 3-6, Sabin 133, and figs. ISS. tsii. Sabin 

 134). This specimen has also a beginning thoracic duet. These two 

 facts show that there is considerable variation in the rate of growth 

 of the lymphatic system of different specimens. 



The study of human embryos has also brought out the fait that 

 the openings of lymphatics into the veins can only lie made out when 



