LYMPHATIC VESSELS. 117 



In order to show the lymphatic vessels, it is generally necessary to 

 stain a tissue with nitrate of silver ; but they may easily be injected by 

 sticking the nozzle of an injecting canula into any tissue which contains 

 them, and forcing coloured fluid under gentle pressure into the inter- 

 stices of the tissue. 1 



In silvered preparations it may be observed that the lymphatics 

 always appear in the form of clear channels in the stained grourid-sub- 



FlG. 142. A SMALL PART OF THE LYMPHATIC PLEXUS ON THE PLEURAL SURFACE OF 



THE DIAPHRAGM. (Magnified 110 diameters. ) (Kanvier.) 



L, lymphatic vessel with characteristic epithelium ; c, cell-spaces of the connective tissue 

 here and there abutting against the lymphatic. 



stance of the connective tissue, and that their walls are in close con- 

 nection with the cells and cell-spaces of that tissue. But, except in the 

 case of the serous membranes, there is no open communication between 

 the lymphatic vessels and the interstices (areolae) of the connective 

 tissue. 



Development of the blood-vessels and lymphatics. The blood- 

 vessels and lymphatics are developed in the connective tissue or in the 

 mesoblastic tissue which precedes it, the first vessels being formed in 

 the vascular area which surrounds the early embryo. Both kinds of 

 vessels are developed from cells (vaso-formative cells or angioblasts) 

 which become hollowed out by an accumulation of fluid in their proto- 

 plasm, and in the case of developing blood-vessels coloured blood- 



1 For details of procedure in injecting blood-vessels and lymphatics the student 

 is referred to the author's Course of Practical Histology. 



