440 ELEANOR LINTON CLARK 



unconnected with the veins, is inappUcable to the superficial lymphatics 

 of the chick. Here a continuous plexus of lymphatic capillaries can be 

 demonstrated by a study of the living, by injection with fine glass 

 cannulae, and by the microscopic study of sections with the oil immersion, 

 hours and often days before any sacs develop. Moreover, the earlier 

 the stage of development of this lymphatic plexus, the narrower and 

 more delicate are the capillaries composing it. The formation of the 

 sacs of the chick is preceded by an active circulation of Ijonph associ- 

 ated with the formation of channels, and although the circulation of 

 lymph is very sluggish in these sacs, it never ceases entirely. The con- 

 nection of the lymph sacs with the rest of the lymphatic system, and 

 through it with the veins, was never lost during the stages studied. 

 Miller^* has represented such a supposedly isolated sac in the jugular 

 region of the chick at a stage (7 days) in which an active circulation 

 through it can be demonstrated, in a living embryo, by injection of a 

 few ink granules into the deep cervical lymphatic channel, which 

 accompanies the jugular vein. 



It has been shown by the researches of Clark and of Evans 

 that the growth of lymphatic endothelium after its differentiation 

 is dependent on its reaction to external stimuli — ^that it responds 

 to substances outside and to the passage of fluid through its 

 wall by the sending out of ne^v capillary sprouts. The present 

 study seems to show that lymphatic endothelium reacts also 

 to the pressure and flow of the fluid inside the walls of the vessels, 

 and that the formation of lymph trunks and of lymph sacs from 

 a primitive plexus represents a response to such stimuli. 



3* A. M. Miller, The development of the jugular lymph sac in birds. Am. 

 Jour. Anat., vol. 12, no. 4, 1912. 



