The Origin and Development of lite Lymphatic System. 31 



budding out, though the internal jugular vein as a whole is formed 

 considerably later. 



For the present the interest in the superficial blood capillary plexus 

 is that it covers the lymphatic ridge. It will be seen in fig. 6 that the 

 lymphatic ridge includes a non-vascular area. This non-vascular area 

 is bounded by the anterior cardinal vein and its dorsal branches, the 

 plexus of the groove, the wide-meshed superficial plexus and the 

 ventro-lateral branches of the anterior cardinal vein. An occasional 

 direct lateral branch of the cardinal vein cuts through the otherwise 

 non-vascular area. Within this area are seen the blood-filled lymphatic 

 buds. They lie in the angle between the anterior cardinal vein and its 

 dorso-lateral branches and connect with both. They are opposite the 

 third, fourth and fifth dorsal segmental veins, and extend a distance 

 of 1.5 mm. from the primitive ulnar along the anterior cardinal vein. 

 The tiny lymphatic buds are already sending sprouts away from the 

 parent vein. 



The presence of stagnant blood, which has proved so valuable a 

 criterion in the living embryo, must be used guardedly in studying 

 sections, unless there is a complete vascular injection. In the lym- 

 phatic area I have noted in the fresh embryo that the blood capillary 

 plexus of the groove is often empty, while the superficial plexus over 

 the lymphatic area and over the pericardium tend to retain some 

 blood. The blood vessels of an area where lymphatics are budding 

 must be thoroughly known before one can be sure of the lymphatic 

 buds. The early lymphatic buds are packed with blood to an extent 

 not common for the veins, which perhaps shows best in figs. 490 and 

 491 (Sabin 134), for the jugular lymphatic buds in a human embryo. 



I have not yet succeeded in injecting the first jugular lymphatic 

 buds in the pig as E. L. Clark has done for the early stages of the 

 posterior lymph heart of the chick. In the embryo 12 mm. long they 

 can be seen in the fresh embryo, and sections show that they have 

 formed a plexus along the margin of the anterior cardinal vein. To 

 see the lymphatic buds at 12 mm. the embryo should be placed in warm 

 Locke's solution while the heart is still beating, and it should be viewed 

 under the high powers of the binocular microscope with the direct 

 sunlight focused upon it. The lymphatic area is wider in the clorso- 

 ventral direction than in fig. 6, but its dorsal boundary is clearly 

 marked by the surface groove. If the superficial blood capillary 

 plexus is empty the lymphatics can then be seen looking like a cluster 



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