The Origin and Development of the Lymphatic System. 51 



Budge on the relation of the heart to the allantoic vessels, have already 

 been given. From that time on there was no advance until the work 

 of Sala (137), who studied the development of the posterior lymph 

 hearts and thoracic duct. He described the posterior lymph hearts 

 as arising in relation to the lateral branches of the first five coccygeal 

 veins, during the seventh day of incubation. We now know that 

 the posterior lymph hearts bud from the veins during the 5th day 

 (E. R. and E. L, Clark, 29), so that Sala was describing the trans- 

 formation of the plexus into the sac. He noted the connections with 

 the veins but described the sac as arising from excavations in the 

 mesenchyme. His work, published a short time before my paper on 

 the origin of the lymphatic system from the veins (Sabin 129), was an 

 advance over the preceding work, for it placed the first lymphatics 

 Bear the veins instead of in the periphery. Sala figured the thoracic 

 duct in the bird as a symmetrical structure (Taf. 14, Fig. 16), and 

 this is also shown in Pensa (104, Taf. 15, Pig. 2). Sala described 

 the early stages of the thoracic duct as being solid cords of cells. This 

 observation, which has been confirmed by Mierzewski (96), makes it 

 necessary to restudy the thoracic duct in the chick, which is now pos- 

 sible through the great improvement in our methods of injection. 



The most recent work on the lymphatic system in birds has been 

 done by Mierzewski (96), Jolly (59), Miller (97) and E. E. and 

 E. L. Clark (27-29). 



I have already brought out the fundamental importance of the 

 Clarks' work, showing that the lymphatics bud off from the veins in 

 a non-vascular zone, and show a continuous growth to the periphery. 

 Mierzewski had shown that the early lymphatics grow along the lateral 

 line of the embryo, as shown in his fig. 3. It was these lymphatics 

 which grow to connect the posterior lymph heart with the jugular 

 lymph sac that E. L. Clark observed in their blood-filled stage and 

 which led her to watch the process of budding in the living embryo. 



The process may best be described in a quotation from p. 254 (29) : 



The first evidence of lymphatics in the tail region of living chicks is the 

 appearance of a number of separate knobs, filled with stagnant Wood, a 

 little darker in color than the circulating blood, just lateral to several of the 

 most anterior of the dorsal intersegmental coccygeal veins. The connections 

 with the veins cannot be seen, since the knobs lie between them and the ob- 

 server, but ink injected into the knobs can be seen to pass directly into the 

 main intersegmental veins. Between the separate knobs no anastomoses 

 can be seen, nor can any be discovered by injection. 



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