232 ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 



form are so rapid that it is very difficult to record them well. By the time a tracing 

 is made with a camera lucida the form is not quite the same and yet the changes 

 can be easily followed with the eye, being not nearly as rapid as the changes occur- 

 ring in the endodermal blisters. Angioblasts may be completely transformed into 

 vessels during the time that the blastoderms are growing on the cover-slip that 

 is to say, in 3 or 4 hours and the process is extensive in one hour. 



In watching a living specimen it can be readily seen that the vacuoles increase 

 in size; many of them flow together and ultimately one may watch the entire center 

 disappear from such a band as is shown in figure 20, plate 4. In figure 25, plate 5, 

 from a chick of 13 somites, can be seen an early stage of the process of the solution 

 of the center of a band of angioblasts. This figure is taken from the angioblasts of 

 the posterior part of the area pellucida. Here and there are large vacuoles just 

 under the edges, while the center of the mass is intact. It will be noticed that some 

 of the vacuoles are against the nuclei, a very common occurrence. I consider it 

 important as showing that the vacuolation or liquefaction is a real intracellular 

 process. The usual conception of the formation of a blood-vessel out of angio- 

 blasts is that the mass breaks up by the separation of the individual cells in the 

 center of the mass, while the cells on the edge flatten out to form an endothelial 

 border. Instead, there appears to be no flattening out of the border cells, but rather 

 vacuoles, which occur just under the borders, leave a rim of cytoplasm while the 

 center of the mass disappears. There is a real differentiation of the cells thus left 

 along the edges of the vessel, because they no longer look just like the original 

 angioblasts. Their nuclei elongate slightly and their cytoplasm becomes less gran- 

 ular, so that in the living specimen endothelium comes to resemble ground glass 

 (figs. 18 and 19, plate 4). These endothelial cells, however, retain the power of 

 reproducing the more granular type of cytoplasm, which they do in giving rise to 

 blood-islands. During this process of liquefaction at the center of these angio- 

 blastic masses, the vessel formed is almost never any larger than the original mass, 

 indicating that the absorption of the fluid present in the surrounding tissue is but 

 slight. 



A later stage in the process is shown in figure 23, plate 5, from a blastoderm of 

 the same stage. In this specimen the center of the mass is disappearing and the 

 vacuolation is much more extensive. Here the edges of the vacuoles are more 

 ragged and little shreds of tissue can be seen in them. On the left border of the 

 mass is to be seen a chain of angioblasts in single file. In this chain the same process 

 is going on between two nuclei, giving the clearest picture of it in a single cell. The 

 same point is shown in the upper right sprout of figure 25, plate 5. There is one 

 place where the process of vacuohzation can be always found within single cells 

 namely, at certain stages over the amnio-cardiac vesicles. Here, as will be de- 

 scribed later, angioblasts always form in long, slender bands (fig. 7, plate 2) and in 

 these bands one can always find the liquef action of chains of single angioblasts. 

 I am quite sure, therefore, that the lumen of a vessel is made within the cytoplasm 

 of a cell and not entirely or even mainly by the separation of cells. It can be 

 readily realized that one often sees the separation of individual cells in such a process. 



