The Origin and Development of the Lymphatic System. 35 



internal jugular vein. This becomes the internal jugular trunk; the 

 cerebral end of this portion is an abundant lymphatic plexus, the 

 anlage of a single gland in the pig (A. H. Clark) which drains the 

 pharynx. In the dorsal lymphatic zone the rest of the sac is a com- 

 plete arch from which develop the lymphatics of the shoulder, head, 

 neck, face, arm and thorax. The dorsal arch of the sac becomes by 

 far its largest portion, as will be readily seen in fig. 8, from an embryo 

 3.5 cm. long. This is the same specimen shown as fig. 3 (Sabin 130). 

 It has since been cleared by the Spalteholz method, so that it shows 

 the sac as well as the superficial lymphatics. 



The transition stages between fig. 7 and fig. 8 are very easy to 

 inject and can be readily imagined. The further development of the 

 sac, especially with reference to the lymphatics along the external 

 jugular vein, is brought out in the sections on the peripheral lymphatics 

 and glands. 



C. THE RENAL SACS. 



The study of the rest of the lymph sacs in the pig may well begin 

 with the interesting and valuable work of Silvester (140). Silvester 

 has shown that in the monkey all of the lumbar and mesenteric lym- 

 phatic ducts drain not by the thoracic duct into the jugular veins, but 

 directly into the renal veins or the surface of the inferior vena cava 

 near the renal veins. This he shows in a number of figures from 

 beautiful injections. 



A. RETROPERITONEAL SAC. 



It had previously been discovered by Lewis (76) in a study of 

 rabbit embryos that a lymphatic sac develops just ventral to the median 

 mesonephritic vein. Baetjer (8) proved that this, the retroperitoneal 

 or pras-aortic sac, arises from the ventral surface of the large vein 

 which connects the two Wolffian bodies in embryo pigs measuring from 

 17 to 23 mm. long. Baetjer's fig. 3, from an embryo 19 mm. long, 

 shows the blood-filled lymphatic buds which have been slightly injected 

 from the veins. These buds rapidly form a large sac which lies in the 

 root of the mesentery' and is the place of origin for all of the mesen- 

 teric lymphatics (Heuer, 43); those that grow to the stomach and 

 intestine, the liver, the capsule of the Wolffian bodies and the repro- 

 ductive glands. The blood-filled lymphatic buds can be seen in the 

 fresh embryo pig 19 to 20 mm. long, if the intestine is pushed to the 

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