480 ON THE ORIGIN AND EARLY DEVELOPMENT 



The point in the development of the lymphatic system at which this lymphatic 

 endothelium becomes the specific tissue present in the stage represented by the 

 lymphatics of the tadpole's tail, and the manner in which this tissue originates, are 

 questions which have been investigated extensively but never completely settled. 



In these observations it has been found that in chick embryos the lymphatic 

 capillaries may be identified as definite endothelium-lined vessels in embryos of 

 4| days, a much earlier stage than any previously described. These early vessels 

 possess endothelial nuclei with morphological characteristics which distinguish 

 them from the nuclei of the mesenchyme cells. The earliest lymphatics are tubes 

 and strands of endothelium, most of them connected with blood-vessels. They 

 have a marked tendency to plexus formation and the resulting plexus in its primitive 

 form is made up of vessels of irregular shape and size, bulbous portions alternating 

 with fine sprouts and solid connections of extreme delicacy. The continuous endo- 

 thelium precedes the formation of a continuous lumen in the primitive lymphatic 

 plexus. Mitotic figures are numerous in these lymphatics, thus showing that the 

 method of growth by sprouting is acquired very early. 



The present studies exclude the possibility that lymphatics are formed from 

 spaces in the connective tissue. The numerous connections between the earliest 

 lymphatics and blood-vessels, the majority of which are lost within the first 30 hours 

 of lymphatic development, make it appear probable that the first lymphatic vessels 

 have arisen from the endothelium of blood-vessels. However, the possibility that 

 the differentiation of lymphatic endothelium may occur in the manner recently 

 described for the first origin of blood-vessel endothelium (Stockard, Sabin) has not 

 been excluded by the methods of study used in this investigation. 



Our studies have also shown that the points of origin for lymphatics are by no 

 means confined to those regions in which lymph-sacs connected with veins are pres- 

 ent in older embryos. The operations on chick embryos have demonstrated at least 

 four regions of origin for the superficial lymphatics. These experiments further 

 show that the character of the blood-vessel endothelium (or mesenchyme) is not 

 specialized in certain limited regions as regards its ability to give rise to lymphatic 

 endothelium; for when a region, normally supplied by ingrowth, is effectually 

 isolated from its sources of supply, lymphatics will eventually develop in situ, but 

 whether from blood-vessel endothelium or mesenchyme cells was not determined. 



