36 Florence R. Sabin. 



side and the veins are emptied of blood by injecting them with salt 

 solution. 



The retroperitoneal sac becomes the largest of all the lymphatic sacs 

 in the pig. As shown in Heuer's fig. 3 (43) it spreads out just behind 

 the rectum and covers the entire area in the root of the mesentery 

 between the two Wolffian bodies. It spreads over the ventral surface 

 of the Wolffian bodies as far as the edge of the reproductive glands. 

 It was from injecting this sac that Heuer was able to study the devel- 

 opment of the lymphatics of the intestine. 



This sac would give an excellent opportunity to study the process of 

 cavernization. Injections of it with silver nitrate give beautiful 

 specimens, showing the endothelial-covered trabecular that cross its 

 lumen. Total preparations of the silvered sac remind one of the trabec- 

 ular in the wall of the cavity of the heart. These trabecular, which 

 are the beginning of the process of transferring the sacs into lymph 

 glands, «how especially well in sagittal sections of pig embryos 20-25 

 mm. long. 



2. Iliac Sac — Cisterna Chtli. 



I can now bring some evidence to show that the iliac lymphatics 

 which drain the legs, tail and abdominal wall, and the cistema chyli 

 which forms the lower part of the thoracic duct, arise together as buds 

 from veins of the Wolffian bodies. A complete account of this process 

 needs a more extensive illustration of the blood vessels of the region 

 than I can give at this time. In the pig the lymphatics which bud 

 off from the veins of the Wolffian body and grow forward dorsal to the 

 aorta to form the cisterna chyli and caudalward along the edge of the 

 Wolffian body to form the iliac lymphatics, do not begin until the 

 embryo is 22 mm. long. I found that they arise from the mesone- 

 phritic veins because a direct puncture of the blood-filled buds entered 

 the main veins and not the blood capillaries. In a litter of pigs which 

 measured 23 mm. I washed out the blood vessels with Locke's solution, 

 and then opened the specimens and pushed one of the Wolffian bodies 

 over toward the midline. A plexus of blood-filled lymphatics was 

 then readily seen dorsal to the aorta. I succeeded in puncturing some 

 of the larger vessels of the plexus forming the cisterna chyli in three 

 specimens and saw the ink run from the lymphatics into the mesone- 

 phritic veins. In all of these injections the cisterna chyli is obscured 

 by extravasations at the point of injection, but they all show the 



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