8 DIRECT GROWTH OP VEINS BY SPROUTING. 



cover-slip it drags the membranes with it. The area pellucida, however, with 

 a rim of the opaca, can be mounted; and although the circulation stops when the 

 embryo is cut away, the cells continue to divide for a short period, so that certain 

 processes can be watched. In such a preparation it was first noted that the 

 granulocytes which develop outside the vessels could wander into the veins, even 

 after a considerable thickness of the adventitia had developed, with just as great 

 ease as they enter the capillaries ; that is to say, the adventitia is no barrier whatever 

 to the wandering of the leucocytes. It was then found that the same was true 

 with regard to sprouting. Sprouts put out from the walls of a vein could push 

 their way between the adventitial cells as easily as through the looser tissue that 

 surrounds a capillary. 



Plate 1 shows examples of such sprouting from veins of the area pellucida 

 in a chick of the fourth day of incubation which was grown for two hours on a 

 cover-slip. In figure A is a long sprout consisting of endothelial cells, for the most 

 part solid, which were growing out from the side of a large vein. It is clear that 

 at the base of the sprout the adventitia is represented by two cells, one on 'each 

 side, that are growing out with the endothelium; that is to say, the vessel is growing 

 as a vein, not as a capillary that is to be transformed later into a vein. Toward 

 the end of the outer endothelial cell is a tiny vesicle, which I think is the beginning 

 of the lumen-forming process. It seems difficult to accept the idea that the lumen 

 of a vessel may develop within the cytoplasm of a single cell, but the process has 

 now been so frequently observed that there is no escape from the fact. 



In figure B is another long sprout from a smaller vein, which shows even more 

 clearly that sprouts grow as veins, for the adventitial cells have wandered even 

 farther along the growing sprout. In this case the lumen of the vein has opened 

 widely into the base of the sprout. On the margin of the main vein there is a 

 considerable heaping up of adventitial cells and several are also seen along the new 

 sprout. The last adventitial nucleus is on the upper side and is the third nucleus 

 from the tip. The branch of the sprout which passes upward has already joined 

 another vein not shown in the drawing. In the new growth of veins one often 

 finds rather large blunt swellings on the side of vessels, like the zone at the 

 base of the sprout in this figure. Such a swelling represents a proliferation of 

 endothelium from which a sprout will eventually form a connection with a neigh- 

 boring vessel. The beginning of this process is shown in figure C, where a group 

 of three endothelial nuclei is to be seen at the base of a short endothelial sprout. 

 This is also a vein, as can be seen from the adventitial nucleus at the right of the 

 base of the sprout. 



Thus from the living specimens is established the fact that not only do the 

 preliminary angioblasts make plexuses by the process of sprouting, but that the 

 resulting capillaries and the veins likewise have this property. The importance 

 of the point concerns (1) the story of how the vessels of each organ develop origi- 

 nally and (2) how to visualize the processes of repair of vessels after injury. If 

 veins can regenerate as veins, it means that we have a much more rational account- 

 ing for the rapidity with which vessels are repaired in wound-healing. In the case 



