The Origin and Development of the Lymphatic System. 23 



from Sala's description that it was this plexus which he described 

 as the beginning of the lymphatics. While the deep lymphatic capil- 

 lary plexus is being transformed into a lymph sac and pulsating 

 lymph heart, lymphatic vessels are already growing toward the skin, 

 where they gradually extend. They grow first along the lateral line 

 to anastomose with the jugular lymphatics. There is a continuous 

 peripheral growth of the lymphatics from the time of the first buds 

 on, and the entire blood-filled stage takes but a few hours. Neverthe- 

 less, it is long enough to enable one to watch the buds form a plexus ; 

 the plexus becomes a sac and lymph heart, and the lymphatics from it 

 spread out in the skin. Injections prove that in every stage of their 

 development, from the primary buds onward, the lymphatics connect 

 with the parent veins; thus there is no separation from the vein and 

 subsequent rejoining, as all of the recent workers, Lewis, Huntington 

 and McClure and myself, have thought from the difficulty of finding 

 the connections with the veins in serial sections. 



The posterior lymph heart in the chick is especially advantageous 

 for study, since it is in the first place sufficiently superficial to be 

 seen in the living embryo, and in the second place it is covered by a 

 non-vascular zone. In mammalian embryos no caudal lymph hearts 

 develop. It is very interesting to note in injected embryo pigs that 

 the corresponding area in the skin at the root of the tail, instead of 

 being a non-vascular zone, is an area of especially wide blood capil- 

 laries. This area, seen in fig. 467 (Evans 35), is constant, and, as 

 it almost always retains a little blood, can be injected when a partial 

 injection of the veins of the lower part of the body is desired. 



The work of the Clarks adds the final proof of the theory that the 

 lymphatics bud off from the veins. Moreover their method is so simple 

 that anyone may convince himself of the essential results by a few 

 hours observations. Their injections have corrected two errors, 

 namely, the idea that the lymphatics are transformed blood capillaries 

 and that they temporarily separate from the veins. 



Thus I regard as proved the three points which have been stated 

 as essential in establishing the origin of the lymphatics from the 

 veins. Stated in reverse order they are : First, the lymphatic system 

 begins as a series of endothelial buds which sprout out directly from 

 the veins as a new type of vessels, namely, lymphatics. These buds 

 unite to form plexuses which develop into sacs. These sacs may become 

 lymph hearts. Second, from these lymph sacs or hearts lymphatic 

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