OF THE LYMPHATIC SYSTEM OF THE CHICK. 465 



II. OPERATIONS ON CHICK EMBRYOS. 1 



Earlier investigations (E. R. Clark, 1909, 1912) have shown that growth of 

 lymphatic capillaries is by a process of sprouting from pre-existing endothelium, 

 while the present study shows that the primitive lymphatic plexus, in the earliest 

 stages of its development, increases rapidly in richness and by this process of 

 growth undoubtedly invades near-by regions. 



Many investigators have considered that the points of origin for the first 

 lymphatics are limited to certain definite regions associated with veins. Thus 

 Sabin (1901) originally described two points of origin for the lymphatics in pigs 

 the jugular lymph-sac and the iliac sacs (both paired). These were later increased 

 by the addition of the retroperitoneal sac (single) discovered by F. T. Lewis (1906). 

 All of the lymphatics of the body were thought to be derived from these points by 

 the process of outgrowth. Other investigators differed from this view, considering 

 that lymphatics have a diffuse origin. Huntington and McClure (1906, 1910) 

 advocated the view that the origin of lymph-sacs is in certain definite regions 

 associated with veins, while believing that other "peripheral" lymphatics arise 

 diffusely from the mesenchyme. F. T. Lewis (1906) suggested that lymphatics 

 arise diffusely from cut-off blood-vessels. Kampmeier (1915) describes the origin 

 of lymphatic endothelium in Bufo as occurring diffusely by outgrowths from venous 

 endothelium at many places. 



We attacked the problem by the experimental method. By operating on chick 

 embryos at a stage before any lymphatics had developed we removed various 

 regions and then studied the modifications produced on the development of lym- 

 phatics. The questions we attempted to solve were the following: 



1 . Are the points of origin for the lymphatic system normally limited or diffuse? 



2. If limited, where are these points of origin? Do they constitute only those regions 

 in which the lymphatics are found to maintain connections with veins (the regions of the 

 "primary lymph-sacs")? 



3. If the points of origin are normally limited, is it possible for lymphatics to develop 

 in situ in other parts of the body when such points of origin have been removed? This 

 would also answer the questions: Is the tissue (blood-vessel endothelium or mesenchyme) 

 from which the first lymphatics differentiate equipotential, or is there some special quality 

 of this tissue in one region which is not found in another? 



In seeking the answers we began by removing the regions from which all of the 

 superficial lymphatics have been thought to arise (Mierzejewski, 1909) the region 

 of the posterior lymph-heart in the tail, and the anterior region near the duct of 

 Cuvier. These are the two regions where lymphatics connecting with veins had been 

 demonstrated in chicks of 5 days and where these venous connections are main- 

 tained throughout embryonic life. The experiments performed were not of a kind 

 to throw any light on the question as to the origin of lymphatics from mesenchyme 

 or from blood-vessel endothelium, since both of these tissues were present before 

 the development of lymphatics in all of the regions studied. 



'The operations on chick embryos were begun in the spring and summer of 1913, in the laboratory of Professor H. Hoyer, 

 at the University of Cracow, Poland. The work for operation 1, and a part of the experiments recorded under operation 2, 

 were completed at that time. It is a pleasure to have this opportunity to thank Professor Hoyer for his cordial hospitality, 

 and for the extremely courteous, friendly, and stimulating interest which he displayed in our work during the very enjoyable 

 months which we spent in his laboratory. 



