30 Florence R. Sabin. 



zone, opposite the third, fourth and fifth segmental veins, that is the 

 lymphatic area. 



To locate the lymphatic area definitely it is necessary to note the 

 details of the superficial veins more carefully. The superficial plexus 

 alono- the lateral groove is finer meshed and of smaller capillaries than 

 the rest. The groove is an important land mark and it shows best in 

 the cross section, fig. 6. From the plexus in the groove a sheet of wide 

 capillaries is growing dorsalward external to the myotomes, and loops 

 from this sheet connect with the deep plexus around the spinal cord. 

 The skin over the entire dorsal surface of the cord is an entirely non- 

 vascular area at this stage. From the ventral border of the super- 

 ficial plexus in the groove a sheet of wider and more open-meshed capil- 

 laries covers the surface of the embryo over the anterior cardinal 

 vein. The surface contour of this, the lymphatic area, is a swelling 

 seen in fig. 6, which I shall call the jugular lymphatic ridge. The 

 blood capillaries of this area are very superficial and drain into the 

 cardinal vein in four ways : (1) Through the plexus in the groove and 

 its dorso-lateral superficial veins: (2) in small part through direct 

 branches to the lateral surface of the cardinal vein; (3) through 

 branches of the ventro-lateral surface of the anterior cardinal vein 

 which drain the pericardium (fig. 5) ; and (4) through anastomoses 

 with the capillaries (fig. 5) of the primitive ulnar vein. The primi- 

 tive ulnar vein has developed frpm the diffuse capillary arm bud 

 plexus of the preceding stage. It enters the lateral surface of the 

 posterior cardinal vein directly opposite the fifth segmental branch. 

 Thus a part of the blood of the branchial region and of the pericar- 

 dium drains through the superficial plexus over the lymphatic ridge 

 into the primitive ulnar vein. In a little older embryos, namely, those 

 12 mm. long, a second vein has developed from the arm bud plexus 

 just cerebralward to the primitive ulnar, which now receives the blood 

 from the plexus of the lymphatic ridge. The complete description of 

 this superficial plexus involves a study of the origin of the external 

 jugular vein which I do not wish to go into until I can illustrate it 

 adequately. I bring up the point here because A. H. Clark (24) has 

 shown that occasionally a lymph trunk in the neck in older pigs opens 

 directly into the external jugular vein near its point of union with the 

 internal jugular vein. It is therefore of importance to note that the 

 capillary plexus in the arm bud, from which this part of the internal 

 jugular vein comes, is present in the embryo when the lymphatics are 



Digitized by Microsoft® 



