The Origin and Development of the Lymphatic System. 39 



and called the posterior sac in 1901 (Sabin 129). The space for the. 

 iliac chain in the pig is small, much smaller than in human embryos, 

 and the iliac vessels in the pig are therefore very inconspicuous in cross 

 sections, except in the blood-filled stage. They can be found, however, 

 when one knows that they lie between the segmental veins and the 

 dorso-medial edge of the Wolffian bodies or kidneys. Sagittal sections 

 are, however, brilliant for the iliac lymphatics and cisterna chyli in 



Wfa 



Mm.. 



M'iy.. 



Pig. 10. — Section through a plexus of vessels filled with blood which 

 marks the beginning of the cisterna chyli in an embryo pig 23 mm. long. 

 (Specimen 23b.) Magnified 40 times. The lymphatics are filled with blood, 

 the blood vessels are empty. A. = aorta; C. c. = cisterna chyli; G. s. s. c. 

 = Gl. suprarenalis s. corticalis; G. s. s. m. = Gl. suprarenalis s. medullaris; 

 M. v. = vena mesonephritica; S. v. = segmental vein; W. b. = edge of the 

 Wolffian body. 



the blood-filled stage. In a sagittal series of a specimen 23 mm. one 

 can see the entire iliac chain in a single section, from the mesone- 

 phritic vein to the blood-filled sac just caudal to the Wolffian body. 

 From this caudal swelling, from which a large group of iliac glands 

 develops in the pig, three sets of vessels grow out: (1) a plexus 

 which completely surrounds the umbilical arteries, (2) the femoral 

 vessels, and (3) the ileo-inguinal vessels. 



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