450 ON THE ORIGIN AND EARLY DEVELOPMENT 



of the lymphatic system in the adult ; a definite idea of the manner of growth of the 

 lymphatic capillary and a few hints as to some of its reactive powers; a picture of 

 the development of some of the component parts of the system, such as the ducts, 

 sacs, and lymph-hearts, and, in some instances, a correlation of their morphology 

 with their function ; a history of its development in many organs of the mammalian 

 embryo and of its differences in pattern and extent in various animals; and a fairly 

 complete idea of its connections with the venous system and of the persistence of 

 such communications in the adult. A more and more complete picture of a definite, 

 independent, and important system has emerged from all these studies, together 

 with a history of the manner in which this system has developed from the earliest 

 beginnings where, however, the outlines of the picture once more become hazy. 

 Since most of this work has been recently reviewed by Sabin (1913 and 1916), it 

 seems unnecessary to enter into a detailed discussion of literature. The work done 

 since that time, and those investigations which bear directly upon the particular 

 problems considered here, will be referred to in the course of the descriptions. 



In the present investigations we have attempted to carry the study back to 

 the first appearance of the lymphatics in the embryo, since this part of the history 

 seemed to be still lacking in clearness. The study involved the consideration of the 

 following unsolved problems: (1) The nature of the earliest lymphatics. (2) The 

 tissue from which they originate. (3) The manner in which the differentiation is 

 accomplished. (4) The question as to how soon the lymphatics form a specific, 

 independent system, whose growth proceeds by the sprouting of pre-existing 

 endothelium in the manner proved for the stage of development of the vessels in 

 the tail of the frog larva (Clark, 1909). (5) The question of the points of origin of 

 lymphatic endothelium in the embryo; whether there are strictly limited areas in 

 which the first lymphatic endothelium arises, or whether the differentiation is 

 diffuse. 



For convenience the description of these investigations and the results obtained 

 have been divided into two parts. Part I deals with the history of the earliest 

 lymphatics in chick embryos and is concerned with the problems of their character 

 and origin. Part II is a report of the operations on chick embryos and is concerned 

 with the question of the points of origin of the first lymphatics. 



