54 Florence R. Sabin. 



There is a rapid extension of these blood-filled structures, and soon, in 

 about an hour and a half after their first appearance, connections between 

 neighboring clusters may be seen. Injection at this stage with India ink 

 shows an anastomizing plexus, connected, as before, with the intersegmental 

 veins. Injection with silver nitrate shows distinct endothelial markings 

 in the walls of the plexus. 



During this plexus formation there is a steady extension toward the sur- 

 face, and by the time anastomoses have formed between neighboring clusters 

 sprouts have grown to the surface and started to extend in the region 

 superficial to the plexus and also ventralwards. It now becomes possible 

 to study with more minuteness the changes which are going on, since these 

 sprouts are quite superficial and are developing in a plane parallel with 

 the surface. Observation and successive records of these sprouts in the 

 living chick reveal a rapid extension ventrally and also anteriorly, ac- 

 companied by a plexus formation. Two or three sprouts are seen to lead and 

 soon numerous connections develop between them. Various portions of the 

 irregular plexus thus formed enlarge and become more densely packed with 

 blood which continues to back up from the vein. Then new sprouts grow 

 out in advance and the same process of extension accompanied by plexus 

 formation is repeated. If a single sprout is selected and frequent careful 

 drawings are made the changes are seen to be rapid and striking. The 

 sprout becomes wider and longer. Branches appear, and they in turn in- 

 crease in width and length. Prom a branch a connection forms with the 

 original sprout, thus forming a loop. New branches and connections are 

 formed, making a plexus. Branches from neighboring loops or plexuses 

 meet one another and anastomose. The several parts of the plexus are 

 quite irregular in size. Most of the lymphatic vessels are several times as 

 wide as. a blood capillary, while some of the connections are as small as, 

 or even smaller, than a blood capillary. Throughout the blood in these new- 

 forming lymphatics is markedly darker in color than the circulating blood. 



This work thus corrects the idea that lymphatics are transformed 

 blood capillaries; They bud out from the veins as lvmphatics and 

 gradually invade the body as a new type of vessels. 



Miller's work (97) on the anterior lymph sac is chiefly valuable 

 in giving an excellent chance to contrast the method of reconstruction 

 with the method of injection in an area where an injection can be 

 obtained. Figure 14 is copied from Miller's fig. 6, and fig. 15 is from 

 an injection by E. L. Clark's (27, fig. 2) of the plexus in the same 

 stage. The points of the apparently isolated islands of lymphatics 

 and the separation of the sac from the veins show by Miller are errors 

 winch are cleared up by E. L. Clark's work. The jugular lymphatic 

 plexus is connected with the vein in a number of places and 'is a con- 

 tinuous endothelial-lined plexus of lymphatics. 



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