250 DR. G. THIN. 



the fact that the spaces which are shortly to become con- 

 nected with the vessels colour more deeply than the other 

 spaces in the tissue, I make the hypothesis that the sub- 

 stance that stains is formed in the blood plasma that has 

 escaped from the vessel. I assume that permeability of 

 the vessel to the fluid elements of the blood has already 

 taken place, and that we have consequently present in the 

 Avidened interstices of the tissue in which it accumulates a 

 plasma which differs from the ordinary lymph fluid. This 

 plasma is the forerunner of the blood current, and when it 

 has sufficiently distended the communication between the 

 spaces and the interior of the vessel, the circulation is prac- 

 tically established. 



These facts receive confirmation from certain injected pre- 

 parations, such as that which is partly shown in figure 1. In 

 this preparation the vessels were completely injected with 

 Berlin blue, the mica square placed some hours in Miiller's 

 fluid, stained in picrocarminate and mounted in glycerine. 

 In most of the milky patches the vascular network was com- 

 plete. In the patch shown in figure 1 the vessels had not 

 formed. The substance of the patch differed, both from that 

 of the patches in which the vessels had formed, and from the 

 non-vascular parts of the membrane, by showing a close 

 network stained by the picrocarminate of such exceeding 

 abundance and intricacy that the attempt has not been made 

 to reproduce it. 



The size of the patch in the figure shows the extent of the 

 staining, and also the distinct manner in which it was sepa- 

 rated from the unstained membrane. There was thus present 

 in the interstices of the patch, even in the smallest of them, 

 a substance that stained in picrocarminate, and this sub- 

 stance had disappeared from the interstices of those patches in 

 whose larger spaces capillary vessels had fully developed. 



By looking at the same figure it will be seen that the 

 injected mass left the capillary at a certain point, and after 

 a short straight course penetrated the larger interstitial 

 spaces of the patch, whose characteristic arrangement can be 

 traced by the coloured injection. There was thus a com- 

 munication between the lumen of the capillary and the in- 

 terstitial spaces before the latter were' permeable for the 

 whole of the contents of the blood-vessels. 



I have purposely avoided discussing the histology of the 

 omentum, except in so far as it bears directly on the subject 

 of development of blood-vessels ; but in order to show the 

 diffierence between the spaces and cells I have represented 

 two branched cells in figure 2, and the nuclei of these cells 



