434 ■ RANDOLPH WEST 



sion of the lymphatics and by no means serve as a test of the 

 process bj^ which this extension is effected once the question of 

 annexation of mesenchymal spaces has been raised. They serve 

 simply as a measure of the process and do not indicate its nature. 



It may be argued that the spaces here described are due to 

 the action of fixing fluids. But if this were so they certainly 

 would not appear only in the region of the embryo in which the 

 lymphatics are developing, and only during the short period of 

 embryonic history during which the lymphatic vessels are formed 

 nor would the border cells of an artefact be flattened to form 

 endothelium. That the spaces exist in the fixed and sectioned 

 embryos, is clearly shown in the accompanying photomicro- 

 graphs, and it is safe to conclude that they exist in the living 

 embryo as well. 



But, is there any evidence that the venous endothelium does 

 not invade this vacuolized tissue and grow out to line these 

 independent spaces? There is: The spaces are in the younger 

 embryos (10.5 mm.) bounded by mesenchyme cells, but as the 

 embryo becomes larger and the spaces increase in size the bound- 

 ing cells become flattened and gradations between mesenchymal 

 cell and endothelial cell are found bounding the spaces (fig. 9, 

 S). Nor is there ever found an endothelial tube within the 

 flattened cells. The idea just discussed has been suggested by 

 Knower (14) without, so far as the writer is aware, the slightest 

 objective evidence in its support. The spaces form and acquire 

 a venous connection so rapidly that in only a few cases are iso- 

 lated spaces found bounded by fully developed endothelial 

 cells, which are disconnected with any preexisting endothelium. 

 But many spaces are found surrounded more or less completely 

 by endothelium and in the remainder of their periphery b}" cells 

 ranging from umnodifled mesenchyme to almost typical en- 

 dothelial cells. 



Mesothelium has been produced experimentally from con- 

 nective tissue, by introducing the factors of pressure and fric- 

 tion, bj^ W. G. Clark (15). He has used non-irritating solid 

 and fluid foreign bodies; celloidin and paraffin, injected into 

 the cornea and subcutaneous tissue, and nnicus which was allowed 



