400 ELEANOR LINTON CLARK 



first proposed by Ranvier, from injections of older embryos, and 

 strongly supported by the investigations of Sabin and others) 

 has been definitely proven by the observations of Clark* on 

 living lymphatic capillaries in the tail of the tadpole. He 

 watched the same region of the same larva for several weeks 

 and observed the sending out of delicate lymphatic sprouts 

 which later acquired a lumen and increased by the mitotic 

 division of the endothelial nuclei. Throughout the period of 

 observation, the lymphatic endothelium remained entirely inde- 

 pendent of the surrounding mesenchyme cells. 



In addition, knowledge of the morphology of the lymphatic 

 system — of the manner in which it invades various parts of the 

 body — has been increased by many investigations. Thus, Miss 

 Sabin and her pupils, by an extensive study of the developing 

 lymphatics of pigs, have given a comprehensive account of 

 the morphology of the system in a mammalian embryo. Hoyer 

 and a number of other workers at the University of Cracow have 

 greatly advanced our knowledge of the comparative anatomy and 

 embryology of the lymphatic system bj^ their investigations of 

 the pattern of the lymphatic capillaries and the position of the 

 main ducts, sacs and hearts in various animals and at differ- 

 ent stages of development. F. T. Lewis, Knower, Stromsten, 

 Huntington and JMcClure, and man}- other investigators, have 

 contributed to this side of the subject. 



On the other hand, questions of the factors which regulate the 

 development of lymphatics, involving a consideration of the 

 reactive powers of lymphatic endothelium and a correlation of 

 the function and structure of early lymphatics have not been so 

 thoroughly studied as in the case of the blood- vascular system. 

 The observations of Clark^ on living lymphatics have shown 

 that the lymphatic capillary reacts continually to external stimuli 

 by the sending out of fine protoplasmic processes, many of which 

 are withdrawn, while others persist, acquire a lumen, and extend 



^ E. R. Clark, Further observations on living growing lymphatics; their re- 

 lation to the mesenchyme cells. Am. Jour. Anat., vol. 13, no. 3, July, 1912. 



* E. R. Clark, Observations on living growing lymphatics in the tail of the 

 frog larva. Anat. Rec, vol. 3, no. 4, 1909. 



