DEVELOPMENT LYMPHATIC SYSTEM IN AMNIOTES 289 



vessels, on the one hand, and of their communication with the 

 veins, on the other hand, the following conclusions: 



1. The formation of the caudal lymph hearts is inaugurated 

 by the development of independent intercellular mesenchymal 

 spaces in the immediate neighborhood of main somatic or appen- 

 dicular venous trunks. 



2. The individual intercellular mesenchymal spaces unite to 

 form a cavity of larger size, and the walls of this cavity are lined 

 by a lymphatic endothelium, developed in situ, by the flattening 

 of the mesenchymal cells along the border of the cavity. This 

 lymphatic endothelium is developed absolutely independently of 

 any other preexisting endothelium, haemal or otherwise. It has 

 no connection with the endothelium lining the blood vascular 

 channels, and it never had such a connection. It is not the 

 result of 'budding' or 'sprouting' or 'proliferating' on the part 

 of any embryonal endothelium whatsoever, but it is simply and 

 solely the result of the response which the embryonal mesenchy- 

 mal cell returns to a mechanical and physical impulse. If two 

 embryonal mesenchymal cells are separated from each other by 

 the accumulation of fluid in the resulting intercellular space, 

 then the opposing aspects of the two cells involved will be sub- 

 jected to the mechanical and hydrostatic influences of accumu- 

 lated intercellular fluid, which will react upon the surfaces of the 

 cell still held in syncytial relation to the surrounding mesenchyme. 

 The cells whose opposing surfaces have become freed by the 

 development of an intercellular space, and are subjected to fluid 

 pressure, will react as a whole, become flattened, and be trans- 

 formed into endothelial cells, forming the parietal limit of an 

 originally intercellular mesenchymal space, which is the font and 

 origin of all vertebrate vascular development, independent of 

 the question as to the ultimate destiny of the space in the organ- 

 ization of the vascular system of which it forms a part. The 

 particular space in question may become a constituent of the 

 heart, of the arterial, of the venous, of the haemal capillary, or 

 of the lymphatic components of the entire vascular complex. 

 This is a matter of individual differentiation in an individual 

 vertebrate embryo. In all four accounts there is tio evidence 



