The Origin and Development of Ihe Lymphatic System. 25 



phritic vein and the veins in the dorso-medial edge of the Wolffian 

 bodies. The lymphatics which grow from the ventral surface of the 

 mesonephritic veins make the large retroperitoneal or prse-aortic sac; 

 those which grow along the dorso-medial edge of the Wolffian bodies, 

 lateral to the aorta, form the iliac sacs, and those which grow dorsal 

 tn the aorta make the cisterna chyli and thoracic duel. The thoracic 

 duet is formed in small part from a dud from the lefl jugular sac; 

 in larger part from a plexus which surrounds the aorta from the 

 cisterna chyli. The retroperitoneal sac drains the diaphragm and the 

 abdominal viscera, except the permanent kidney; the iliac sacs drain 

 the permanent kidney, the abdominal walls, the hindlegs, tail and em- 

 bryonic membranes. 



B. THE .11(11 I. Alt SACS. 



As has already been stated in part 111, the spreading of the primary 

 superficial lymphatic capillaries in the skin of the embryo pig takes 

 place while the embryo is growing from 20 to 50 mm. long. The first 

 lymphatic buds are found when the embryo is aboui 1 1 mm. long 

 (fig. 6); the sac is well formed at is mm. (fig. ]). and reaches its 

 maximum size in an embryo 30 mm. lone (»fig. 8). It remains as a 

 large sac until the embryo measures -"it) mm., when it begins to be 

 transformed in pan into lymph glands (fig. 16). 



An understanding of the development of the jugular sac depends 

 on a knowledge of the veins of the region. Indeed. 1 was not able to 

 inject the early stages until 1 had so mastered the pattern of the 

 blood vessels that a little bl 1 in any of them enabled me to repro- 

 duce the whole picture and so avoid the blood vessels. 



The study of the form of the earlv anterior cardinal vein is shown 

 for the chick by Evans (34) in his figs. 1 to '■'>. These figures show 

 that the primitive deep dorsal segmental branches drain the ventro- 

 lateral surface of the spinal cord. They bring out the gradual curve 

 of the anterior cardinal vein and the more marked bend of the pos- 

 terior cardinal vein, where the two join the duct of Cuvier. This 

 stage is shown for the chick in cross section by Evans (35) in his 

 fig. 437 B, as well as in ( Evans 34) his fig. 4. 



In fig. 4 is given an injection of the blood vessels in an embryo pig 

 "t mm. long, a stage just before the lymphatics begin. The curve of 

 the dorsal border of the cardinal veins toward tin duct of Cuvier is 



