ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 239 



they are anchored out into the lumen of the capillaries by numerous guy ropes of 

 cytoplasm. Instead of developing hemoglobin like the analogous cells of the 

 embryo,- they develop phagocytic powers to a high degree. 



These sprouts of cytoplasm, proceeding from both the angioblasts and the cells 

 of the blood-islands within the lumen of vessels, are referred to many times in the 

 literature as evidence of amoeboid activity. In reality, both types of cells have but a 

 slight power of movement from place to place, and the sprouts represent a rather 

 remarkable degree of attraction of similar cells. By means of these processes 

 isolated masses of angioblasts are brought into a plexus, compound blood-islands 

 are formed, and blood-islands are, as it were, more securely anchored to the inner 

 wall of a vessel. 



In figure 18, plate 4, a certain number of erythroblasts are seen in the lumina 

 of the vessels. These cells, which have become free from the islands, continue to 

 divide, even while they are circulating. These circulating cells make it quite easy 

 to determine the lumen of a vessel in the living chick, and, after studying the blasto- 

 derms in which the circulation has been established, one will never have any difficulty 

 in identifying the lumen. In the specimen shown in figure 12, plate 3, blood was 

 being pumped into the area of the omphalomesenteric arteries, which are just 

 beginning to be indicated in the figure, but there was no movement of the blood in 

 the area represented in figure 18, plate 4. In figure 19, plate 4, is shown a later 

 stage in the formation of blood-islands. This was also drawn while the cells were 

 in the phase of division, and hence each cell in the mass stands out individually. 

 In the fixed specimen the nuclei of more than half of the cells are in the prophase of 

 division. In the resting stage of these older islands the cells are no more definite 

 in the center of the mass than is shown in figure 18, plate 4. The border of the 

 islands, however, now displays the contours of the individual cells instead of the 

 sharp, smooth contour of figure 18. If islands, such as the one shown in figure 19, 

 plate 4, are watched in the living specimen one will see the cells (one after another) 

 free themselves from the edges. This process is surprisingly slow. I have seen it 

 take 1| hours for a single cell to become separated from an island. 



These preparations give a good chance to test out the idea as to whether the 

 primitive mesamceboid cells are really amoeboid at all. The freeing of an individual 

 cell from an island of course involves power on the part of the individual cells to 

 move, but as seen in the living form this movement is very slow and not associated 

 with much, if any, change in the shape of the cell. Moreover, a cell that has just 

 become free from an island, provided there is no current fluid by which it can be 

 carried away, will stay close to the island where it was formed, and one has to watch 

 closely to detect the slight changes in its contour. This is very different from the 

 rapid changes characteristic of the white corpuscles. I am of the opinion that the 

 marked, blunt processes which are found in specimens of erythroblasts of young 

 chick embryos are associated with a reaction to the concentration of the fluid in 

 which they are placed, because these processes can be much increased by simply 

 increasing the concentration of the sodium chloride in the solution. I therefore 

 conclude that the young erythroblasts are sensitive to osmotic pressure, and that 



