52 ADMONT H. CLARK 



as shown in figure 1 the lymphatics for the skin of the neck and 

 the submaxillary vessels, while the caudal end of the plexus gives 

 rise to the superficial lymphatics of the arm and of the thoracic 

 wall. Thus the jugular lymph sac gives rise to the suprascapular, 

 occipital and pharyngeal lymphatics directly and is the place of 

 origin of the superficial cervical plexus which in turn supplies 

 all the rest of the lymphatics for the head, face, neck, thorax and 

 arm. The deep lymphatics of the arm have not yet been worked 

 out in the pig. In the cat and in human embryos they arise from 

 an extension of the jugular lymph sac which lies along the primi- 

 tive ulnar vein. 



These fundamental groups of lymphatics the suprascapular, 

 occipital and superficial cervical, which can be seen in the embryo 

 3.5 cm. long and indeed can be injected a short distance from 

 the lymph sac much earlier namely in specimens measuring 18 

 mm. are constant. Ducts originate from definite places and 

 establish definitely-localized plexuses. Thus in the early stages 

 there are distinct plexuses in the skin which are connected with 

 each other only through their central connection with the sac. 

 Such a primary plexus for example is the occipital plexus of figure 

 2. By subsequent development however, these separate areas 

 become interconnected, so that an injection instead of being lim- 

 ited to one of the primary plexuses spreads out quite widely, reach- 

 ing the sac not by a single set of ducts but by a number accord- 

 ing to the extent of the injection. Thus, plainly, the earliest 

 lymphatics drain definite areas which are distinctly located and 

 definitely defined. 



In figure 2 it is shown that the suprascapular vessels drain 

 by a few vessels sometimes not more than one or two directly into 

 the apex of the sac. At this stage there are a few small anasto- 

 moses between the suprascapular vessels and the sui:)erficial 

 cervical plexus. These anastomoses are destined to become 

 very abundant so that there are eventually more vessels which 

 connect the suprascapulai- jilexus with the superficial cervical 

 plexus than with the primary sac. 



This process of the development of anastomoses between the 

 <lifforent primary plexuses goes on until the entire superficial 



