DIRECT GROWTH OF VEINS BY SPROUTING. 9 



of the healing of the vessels in intestinal anastomosis, we know that vessels from 

 one of the apposed surfaces of the intestine can be injected from the other surface 

 on the fourth day after the operation. If veins can grow as veins, the reestablish- 

 ment of the circulation can doubtless be more rapid than by a process of the pre- 

 liminary development of a capillary bed out of which the larger vessels must 

 subsequently form. 



Along with the processes of growth in these living specimens, it is possible also 

 to follow the important subject of the destruction of vessels. In the area vasculosa 

 there are regions in which one finds an extensive plexus of capillaries followed a 

 short time later by a stage in which the same area has only one or two large vessels. 

 A most interesting place to follow such a change is in the origin of the main vein, 

 which develops to accompany the primary stem of the omphalo-mesenteric artery. 

 Such a transition must involve a destruction of vessels and one should be able 

 to follow this process in a living specimen. Figure D is taken from the same 

 blastoderm as the other figures, but shows veins which were disappearing rather 

 than growing at the time the specimen was fixed. All of the other figures were 

 near together in a growing zone, while this figure is taken farther along the course 

 of the same veins, where branches were degenerating. The main large vein at the 

 right of the figure is normal. From this vein are two branches in which both the 

 endothelial and the adventitial cells are to be seen in a stage of advanced degener- 

 ation. The cells are full of vacuoles and granular detritus and lead over to another 

 smaller vein on the left side. The specimen shows clearly that the first stage in 

 the degeneration of a vessel is a preliminary collapse of the endothelium which 

 obliterates the lumen of the vessel. The evidence for this is a solid core of endo- 

 thelium in a structure that was a vein. This is probably an important step in 

 preventing hemorrhage during the degeneration of vessels. In this specimen the 

 next stage is the death of the cells, both endothelial and adventitial. It seems to me 

 possible that in some cases there may be a retraction of the endothelial sprouts, 

 after the collapsing of the lumen, instead of actual death of the cells, making the 

 process the reverse of the sprouting which characterises the growth of vessels. If 

 this takes place, it should be possible to find it in a living blastoderm, but so far it 

 has not been observed. As a matter of fact, the methods of destruction of vessels 

 in a growing zone are second in interest only to the methods of spreading of vessels, 

 so often are the vessels formed and re-formed before the final pattern is reached. 



It seems to me clear that the work of the past twenty years on the develop- 

 ment of the vascular system has established its fundamental genesis and has given 

 us the broad outlines on which the story of the spread of the vascular system 

 over the body has become a feasible problem. Instead of lessening the interest 

 in the problem, as one for which we can now see a conclusion, the whole subject 

 has rather been opened up to a new experimental attack by which we may hope 

 to analyze more deeply some of the factors in development that control and modify 

 the system. 



