50 Florence R. Sabin. 



13 and 14 mm. long there is an abundant plexus of capillaries anterior 

 to the junction of the primitive ulnar vein with the internal jugular 

 vein, readily injected from the veins. In embryos slightly older 

 this plexus of capillaries is being transformed into a sac, and these 

 sacs are less readily injected from the veins. For example in an 

 embryo 15 mm. long, the sac was injected on the side from the vein 

 and not in the other. About this time then the primary connections 

 with the vein become severed. La my specimens the sacs are filled 

 with blood. When the secondary opening into the veins is formed 

 the sacs become empty and this is true in pig embryos 16 mm, long. 

 In connection with human embryos I shall show how to determine 

 the presence of these secondary openings or valves. 



This method of formation of the jugular sacs was also confirmed 

 by Huntington and McClure in studying cat embryos. ^^ They have 

 followed all the details of the transformation of the simple veins 

 to the abundant venous plexus and the sac formation by Born's 

 method of reconstruction. Thus the origin of the jugular sac has 

 been worked out in the pig, the rabbit and the cat by the methods of 

 injection and of reconstruction both in two and in three dimensions. 

 The formation of this jugular sac will also be illustrated in the human 

 embryos in this paper. 



Besides the jugular sacs two other paired sacs and two unpaired 

 have been described. Lewis described symmetrical subclavian sacs 

 in the rabbit, which in human embryos are, however, an extension of 

 the jugular sacs; the other paired sac is the posterior or sciatic 

 one, noted in the pig and more fully marked out in this paper for 

 human embryos. The unpaired sacs are the cisterna chyli and the 

 mesenteric or better retroperitoneal sac. 



This sac was discovered by Lewis its origin and development have 

 been worked out by Baetjer ; its significance is brought out in 

 Heuer's work in connection with the lymphatics of the intestine. 

 Mr. Baetjer^'* has shown conclusively that the retroperitoneal sac 

 begins as a series of small veins which bud off from the renal veins. 



In his figures are shown the small veins in the root of the mesen- 



"Huntington and McClure. The Anatomical Record, Vol. II, 190S. 

 "Baetjer. Amer. Jour. Anat., Vol. VIII. 



