OF THE LYMPHATIC SYSTEM OF THE CHICK. 451 



I. THE HISTORY OF THE EARLIEST LYMPHATICS IN CHICK EMBRYOS. 



In approaching this phase of the problem we decided to select some one region 

 of the embryo and to study it intensively by all available methods, assuming that 

 the manner of formation of the earliest lymphatics follows the same laws in other 

 regions. The posterior lymph-heart region was chosen because of its accessibility 

 for observation in the living and for direct injection, and because previous work 

 had made us familiar with it. 



The posterior lymph-heart of the chick was discovered in 1882 by Budge, and 

 various stages in its development have since been described by Sala (1900), Mierze- 

 jewski (1911), E. R. and E. L. Clark (1912), and West (1915). The region of the 

 posterior lymph-heart is situated on either side of the tail, beginning at the angle 

 formed by the posterior border of the pelvis and the tail, and superficial and lateral 

 to the myotomes. In chicks of 8 days the lymph-hearts in this region are clear, 

 almond-shaped bodies, easily visible in the living embryo (as stated by Sala and 

 by Budge), and their pulsations can be noted. Sala found that the lymph-hearts 

 communicate with the intersegmental coccygeal veins, connecting at first with five 

 of these veins and later with three. By the tenth day the hearts are rounded in 

 form and surrounded by a layer of fat. Budge injected the lymph-hearts from the 

 allantoic lymph-vessels in chicks of 9 days. He found that they were present in the 

 chick until after the time of hatching, but were absent in adult chickens. Stannius 

 (1843) and Panizza (1830) had previously shown that lymph-hearts are present in 

 adult aquatic birds as well as in the ostrich and cassowary. 



In studying earlier stages of the lymph-heart Sala made use of cross sections 

 of chicks of 6 to 7 days old. In these he describes a series of "spaces" connected 

 with the intersegmental coccygeal veins and concludes that the lymph-heart is 

 formed by the coalescence of these spaces; at one time he refers to them as mesen- 

 chymal spaces and at another as continuations of the veins themselves. 



In 1901 Sabin published her paper describing the embryonic lymphatic system 

 of the pig, which, in the earliest stages, consisted of paired jugular and iliac sacs 

 and the thoracic duct connecting them. These sacs were connected with veins and 

 seemed to be the homologues of the lymph-hearts of lower animals, and Sabin 

 concluded at the time that they were outgrowths from the veins and were the 

 primary points of development for the lymphatic system, from which all other 

 vessels were derived by a process of outgrowth. This view that the lymphatics 

 grow by sprouting had been advocated by Ranvier (1895) from studies made on 

 much older embryos. 



In 1906 F. T. Lewis carried the history of the lymphatic system back a step 

 farther by his studies of rabbit embryos. In the region of the jugular lymph-sac, 

 and before the appearance of that structure, he found a discontinuous plexus of 

 capillaries liried with endothelium. From the study of fixed material he suggested 

 the possibility that these were "veno-lymphatics" or blood-vessels, formerly part of 

 the general circulatory system, which had become cut off and were about to be 

 transformed into a lymph-sac. Huntington and McClure (1910) described a stage 



