234 ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 



of the heart, in the zone in which the vitelline veins are developing. In a chick of 

 8 or 9 somites it can almost always be found going on in the lower end of the aorta, 

 opposite the undifferentiated mesoderm and just posterior to the somites; while 

 chicks with from 12 to 14 somites will nearly always show the process in the pos- 

 terior zone of the area pellucida. This zone is the best in which to observe it in the 

 living form. 



From these observations it appears to be clear that the lumen of a vessel is pro- 

 duced by the liquefaction of the central mass of angioblasts to make blood-plasma, 

 and that while whole cells are destroyed and some of the original cells may separate 

 from the original mass, the essential process is intracellular and the formation of the 

 blood plasma by liquefaction is one of its important effects. This fact is brought out 

 most strikingly when the lumen of a vessel is formed in chains of single angioblasts 

 and where there can be no question of the lumen developing in potential clefts 

 between cells, since it is within the body of a single cell. The same fact is actually 

 shown by noting that many of the vesicles begin against the nuclei in the syncytial 

 masses. Thus the first blood-plasma is the result of the destruction of large masses 

 of angioblasts by a true cytolysis, and one of the important functions of these 

 primitive masses of angioblasts is the formation of plasma. Since the tissue fluid 

 is formed before the vessels begin, the endothelium, from the beginning, is a mem- 

 brane which separates two different fluids, plasma and tissue fluid. 



As has been said, the process of liquefaction of protoplasm is shown in section 

 in figure 28, plate 6, which is from a vessel just under the edge of the area opaca of 

 a chick of 11 somites. In this section the center of the larger mass has almost dis- 

 appeared, while the endothelial border is seen full of tiny vacuoles. In the upper 

 part of the section is a blood-island. The lower vessel, on the other hand, shows 

 again the liquefaction of the angioblasts beginning along the edge of the main mass. 

 The endoderm in this section is also interesting, showing both the syncytial net- 

 work of the cytoplasm and some of the wandering endodermal cells which are 

 becoming free from the network. 



The idea that the lumen of a vessel can be considered as intracellular is not 

 new. It was first expressed by Strieker (1866), who conceived the idea from 

 studying the tiny sprouts to be made out along the capillaries in the tail of the living 

 tadpole. Concerning it, he says: 



" Wenn ich sage, die Wande der Capillargefasse sind Protoplasma, dann muss ich wohl 

 selbst zugeben, dass sie aus Zellen bestehen; nichts destoweniger liegt ein tiefer und durch- 

 greifender Unterschied zwischen dem, was uns an den Larven klar und unwiderleglich vor 

 Augen tritt, und zwischen dem, was in neuster Zeit aus der .Silbermethode deducirt wurde. 

 Nach dieser Deduction sollen die Capillaren aus Zellen zusammengefugt sein, und das 

 Blut in jenen mithin intercellular fliessen; nach dem was sich an der Larve ergiebt, ist ein 

 Capillarrohr Protoplasma in Rohrenform, Protoplasma, welches im Innern aushohlt ist, 

 und wo das Blut gleichsam intracellular fleisst" (page 7). 



It was then brought out in a series of interesting papers published in the early 

 seventies. As has been said, it was most clearly and correctly presented by Klein 

 in 1871, and later was expressed by Balfour (1873), Schaefer (1874), Ranvier (1875), 



