48 ADMONT H. CLARK 



The work of Sabin has shown that lymphatics first appear in 

 the embryo pig 10 to 11 mm. long as an outbudding from the ante- 

 rior cardinal veins opposite the third, fourth and fifth segmental 

 branches. From these primitive buds a plexus of lymphatics is 

 formed along the dorsolateral border of the anterior cardinal 

 vein and this plexus is transformed into a non-muscular endothe- 

 lial lined sac. From this primitive sac by continued centrifugal 

 growth a large number of sprouts grow dorsalward into the pos 

 terior triangle of the neck and form a complete arch of lymphatic 

 capillaries connecting at either end with the primitive sac. This 

 entire arch of capillaries becomes transformed into a part of the 

 jugular lymph sac which explains the form of the final sac as shown 

 in figure 1. From the jugular sac, still by centrifugal growth, 

 the peripheral lymph vessels radiate forward over the head and 

 backward over the anterior part of the body forming plexuses 

 which are characteristic and definitely located .^ 



For convenience I shall use the following terms in referring to 

 the Ij^mph sac, the form of which is shown in figure 1. (1) The 

 anterior curvature of the lymph sac is the portion lying behind 

 the pharnyx against the internal jugular vein. (2) The sac 

 stalk is the portion of the sac also on the internal jugular vein 

 extending between the point where the valve develops at the 

 junction of the internal and external jugular veins and the ante- 

 rior curvature. This is the first part of the sac to develop. (3) 

 The apex is the portion of the sac lying in the posterior triangle of 

 the neck. The reasons for this division of the sac are not obvi- 

 ous in figure 1, but I shall show that they correspond to the func- 

 tion of the three different parts of the sac. The apex of the sac 

 connects with the sac stalk both through the anterior curvature 

 and more directly by a large channel which joins the stalk not 

 far from the valve into the vein. 



The form of the jugular sac is well shown in figure 1, which is a 



'diagram made from an embryo pig 2.8 cm. long and in figure 2 



which is a drawing of an injection of the lymphatics in a pig 3.5 



cm. long. From the sac four groups of lymphatic vessels develop. 



^ A part of this work is in the Amer. Jour. Anat., vol. 1, 1901-1902, and a part of 

 it will be published in the Ergebnisse fiir Anat. und Entwicklungsgeschichte. 



