DEPARTMENT OF EMBRYOLOGY. 115 



nuclei of new-forming sprouts. Frequent mitotic figures may be seen 

 in the endothelium of early lymphatics. 



The nucleus of the mesenchyme differs in all the particulars men- 

 tioned. It contains two or more nucleoli which are not sharply dif- 

 ferentiated from the remainder of the chromatin material of the nucleus, 

 but which extend out into prongs and threads, and it does not have 

 a characteristic shape. It stains distinctly bluish. The remainder of 

 the nucleus has a shghtly darker appearance and often contains small 

 clumps of chromatin material. 



The fact that the nucleus of the lymphatic endothelium possesses 

 distinct morphological characteristics different from those of the mesen- 

 chyme cells in chick embryos, and that these characteristics are present 

 in the earliest recognizable stages of lymphatic development, coupled 

 with the fact that the nuclei of blood-vessels, with which the lymphatics 

 are connected, have quite similar characteristics, furnish important 

 evidence that the lymphatic endothelium is derived from the veins. 



Our investigations concerning the development of the lymphatic 

 system have been under way for several years and center about the 

 work of Professor Florence R. Sabin. The purpose of her investiga- 

 tions has been to determine the morphology of the system as a basis 

 on which to work out its development and its relation to other sj^stems. 

 Her study began with the discovery of centrally placed jugular sacs, 

 which lie on the veins and constitute the first lymphatics. This dis- 

 covery reversed the theory accepted at that time, namely, that the 

 lymphatics began in the periphery and grew centrally. Starting, then, 

 on the theory that the lymphatics develop from sacs, which in turn 

 arise from veins, it was shown that they gradually grow outward and 

 invade the body. This gradual invasion of the body involves, there- 

 fore, the study of the development of the entire system up to the adult 

 stage. The principles of the development have been established, but 

 the details have been only in part worked out. The next crucial point 

 in this study was that relating to the nature of the growing lymphatic 

 tips. That the growth of the lymphatic tip is by an extension of its 

 own endothelium was demonstrated by the method of injection and 

 observation of sections as far as was possible bj^ those methods. The 

 demonstration was put on an incontrovertible basis by Professor Eliot 

 R. Clark, by observations on the growth of the lymphatic tips in the 

 living tadpole's tail, in which he showed that new lymphatics invariably 

 arise through growth from preexisting lymphatics. The next step in 

 the chain of proof we owe to the work of Eleanor Linton Clark, who 

 discovered, in the living chick, the blood of the vein backing into the first 

 lymphatic buds, thus showing that the first lymphatic buds communi- 

 cate constantly with the veins and therefore must arise from them. 



Thus it has been shown that the lymphatics arise from the veins, 

 grow by sprouting, and gradually spread over the body. 



