404 ELEAXOR LINTOX CLARK 



lymphatic injections with India ink, the principal blood vessels 

 had been injected with aqueous Berlin blue. 



After the 10th day of incubation, the increased thickness of 

 the skin and the development of feathers prevented further 

 observation of the circulation of granules in the superficial 

 lymph vessels. For this reason, the present study is confined to 

 the changes in the structure and function of the superficial 

 lymphatics during their development in chicks of 5| to 9 days. 



THE PRIMITIVE CONDITION OF THE SUPERFICIAL LYMPHATICS 



■J 



The primitive form of the early superficial lymphatic system, 

 in the chick, is that of a richly anastomosing capillary network 

 which spreads out over a large area. This plexus communicates 

 with the venous system at certain points but remains independent 

 of the surrounding plexus of blood capillaries from which it can 

 be distinguished in living embryos by its more irregular form and 

 by the more darkh^ colored stagnant blood normally present in 

 its vessels. Each set of capillaries (blood vessels and lymphatics) 

 can be injected separately without disturbing the other. The 

 vessels composing the primary lymphatic plexus vary in size and 

 shape, large knob-like portions alternate with delicate con- 

 nections, finer than blood capillaries, and small pointed pro- 

 jections. That these early l^inphatics are lined throughout 

 with endothelium is evident from the silver markings which 

 show distinctly after the plexus has been injected with silver 

 nitrate. The continuous endothelial lining can also be seen in 

 microscopic sections studied with the oil immersion, where E. R. 

 Clark ^ has shown that the endothelial nuclei have definite 

 morphological characteristics which distinguish them from the 

 nuclei of the surrounding mesenchyme cells. 



Not all the points of origin for the lymphatics of chicks have 

 been determined. One of them, the region in the tail lateral 

 to the myotomes, a portion of which is occupied at a later stage 

 by the posterior lymph heart, has been studied in detail by Dr. 

 Clark and myself and will be reported later. 



^ E. R. Clark, On certain morphological and staining characteristics of the 

 nuclei of lymphatic and blood-vascular endothelium and of mesenchjine cells 

 in chick embryos. Proc. Amer. Assoc. Anat.. Anat. Rec, vol. 8, no. 2, 1914. 



