OF THE LYMPHATIC SYSTEM OF THE CHICK. 463 



Although the possibility that the first lymphatics may arise from mesenchyme 

 cells is not excluded by these studies, their differentiation from mesenchyme spaces 

 appears to be clearly untenable. These vessels appear at a time when the sur- 

 rounding tissue is uniformly dense. The more open tissue directly adjoining the 

 muscle, which in chicks of 4^ to 5 days contains spaces, is not invaded by the devel- 

 oping lymphatics. The stages showing the loose tissue containing the spaces form- 

 erly thought by some writers to be "pre-lymphatics," were found to be stages in 

 which there is already a luxuriant plexus of lymphatics whose continuous endo- 

 thelial lining and total independence of the spaces were demonstrated by all the 

 methods used for these investigations. The isolated "pre-lymphatic" spaces, some 

 of which were endothelial-lined and others apparently not, are quite evidently the 

 result of incomplete reconstruction, inevitable when uninjected material and low 

 powers of the microscope are used. 



The first lymphatics occur in a dense region in which spaces are conspicuously 

 absent, and the primitive plexus spreads out regardless of the character of the 

 tissue which it invades. Everywhere this earliest plexus has the same form, con- 

 sisting of delicate, string-like processes, minute thread-like connections, and larger, 

 rounded, nodal points, the latter probably representing the result of beginning 

 absorption. The plexus appears to be only secondarily influenced by the character 

 of the tissue in which it is situated, and in chicks of 5 days and over those portions 

 situated in a looser area, such as the lymph-heart region and the region anterior to 

 the hind-limb, have larger vessels than those located in dense regions, such as the 

 posterior tip of the pelvis and the axilla. 



It is, of course, a great disappointment that we were unable, even by the use 

 of such a painstaking method, to carry the history of the first lymphatics to a point 

 at which the mode of origin of the lymphatic system could be established beyond a 

 doubt; but we believe that the present studies have yielded valuable information 

 with regard to the nature of the earliest lymphatics in stages in which the existence 

 of such vessels had not been suspected. It has been possible to show that these 

 vessels have definite morphological characteristics which distinguish them from 

 other tissues. Also, the form of the beginning lymphatic plexus, with its many 

 solid processes and its numerous mitoses, makes it appear that the method of 

 growth of lymphatic capillaries, described by Clark (1909, 1912) for the trans- 

 parent tails of amphibian larvae, is also the method of growth of the primitive 

 lymphatic system. In other words, the specificity of lymphatic endothelium has 

 been traced back to a very early stage. 



The history of the lymphatics of the chick in the region of the posterior lymph- 

 heart has been traced back from the 8-day stage described by Budge (1882) and 

 Sala (1900), in which a definite pulsating lymph-sac is present, to the stage of 6| 

 days in which the pulsations begin, although the heart is still in the form of a 

 plexus; then to the stages of 5 to 6 days, in which a luxuriant blood-filled lymphatic 

 plexus is present; to a still earlier stage in which a continuous plexus of finer vessels, 

 without a continuous lumen, occupies this region; and finally, to a stage in which 

 can be found a few very delicate vessels, most of which are connected with blood- 

 vessels, and which have not yet formed a plexus. 



