LYMPH-FLOW AND LYMPHATICS, CHICK EMBRYOS 405 



In chicks of 5 days, the superficial lymphatic system con- 

 sists of a plexus of very fine vessels and is confined to two local- 

 ities — ^the region lateral to the myotomes on each side of the tail, 

 where certain portions of it connect with the first five coccygeal 

 veins, and also to the neighborhood of the thoraco-epigastric 

 vein. At this stage, the anterior plexus extends along the side 

 from the axilla as far posteriorly as the beginning of the leg. 

 When it is injected and the embryo cleared, a continuation 

 from it can be seen which runs through the axillary region and 

 connects with a deep plexus of vessels located just dorsal to the 

 anterior and posterior cardinal veins, near their junction to form 

 the duct of Cuvier. The deep plexus in turn communicates with 

 these veins at a number of places. 



The primitive lymphatic plexus is characterized by a wild 

 luxuriant growth and it extends rapidly, so that in a chick of 

 about 5 days and 20 hours, a continuous superficial plexus of 

 Ijrmphatic capillaries can be injected over the whole side of 

 the embryo, from the axilla to the tail. The size of the lym- 

 phatic capillaries of this early plexus varies according to the 

 character of the surrounding tissue. In general, most of the 

 vessels are larger and more distended in the lymph heart region 

 and in the region just anterior to the leg, where the tissue is 

 loose, while they are finer over the posterior part of the pelvis 

 and in the axilla where the tissue is denser. 



During all this primitive stage, the lymphatics normally 

 contain stagnant blood. In one of the preliminary reports 

 referred to, I stated that these blood-filled lymphatics were non- 

 functioning. This statement now appears to have been some- 

 what inaccurate for it is quite probable that the very abundant 

 formation of new capillaries which characterizes these early 

 lymphatics is associated with the passage of fluid through their 

 walls. Rather, it should be said that the stage in which stagnant 

 blood is present in the early lymphatics is a stage in which there 

 is no circulation of lymph. 



It is evident from a number of observations and experiments 

 made by Dr. Clark and myself, which can only be referred to 

 briefly at this time, that the blood backs up into the early 



