54 ELIOT R. CLARK 



soon as the Berlin Blue mixes with the blood it forms a precipi- 

 tate which plugs the vessel and sticks to the endothehal wall, 

 outlining the position of the vein. Using this as a guide, the 

 vessel is dissected out, with considerable of the surrounding tis- 

 sue, to make certain that all is removed. The egg is then sealed 

 and the chick allowed to develop further. 



This experiment was performed successfully six times and 

 in every case, there was found to be a large vein in the place 

 of the one removed. In one case, the vein on the operated side 

 was larger than the one on the unoperated side. The chicks 

 were examined four to eight days after the operation. 



The conclusion seems justified that the secondary develop- 

 ment of a large vein in the neck, in the place of the one removed, 

 indicates that the mechanical conditions of the circulation favor 

 the growth of a large vein in this region. Surely the new vein 

 can hardly be considered as due to inheritance. 



What are the laws which govern the growth of the endothe- 

 lium, making the differentiation of such an elaborate system pos- 

 sible? What, in other words are the modes of reaction of blood- 

 vascular endothelium? 



THE FORMATION OF NEW CAPILLARIES 



The first property to be noted is the capacity of the endothe- 

 lium to send out sprouts. This process has been frequently ob- 

 served in the transparent tails of living frog larvae, and verified 

 by other studies, and is the generally accepted mode of spreading 

 of the vascular system, after its primary differentiation. The 

 sprout consists of an elevation of the endothehum which is sent 

 out, usually starting at right angles to the vessel wall, and with 

 a lumen continuous with the lumen of the parent vessel. The 

 end of the sprout consists of a solid process of varying length, 

 which may be in the form of a single thread, or of a thread with 

 one or more branches. This process usually extends in a straight 

 line from the parent vessel, for a varying distance, and may then 

 curve. Sooner or later it reaches a similar sprout, or approaches 

 a fully formed capillary, when it shows itself possessed of a prop- 



