ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 237 



blasts, it is the angioblasts that make the striking feature of a specimen, as shown 

 in figure 20, plate 4. After the lumen has formed it is the exact reverse, for then 

 the interspaces are conspicuous and simulate vessels. This, I think, will be clear 

 from a careful examination of the photograph shown in figure 11, plate 3. In this 

 figure there are pale rings in the area pellucida, showing especially well at about 

 the middle of the area. These rings are the interspaces and the dull network between 

 them are the vessels. The six sharply outlined spots near the undifferentiated 

 mesoderm, posterior to the somites, are defects in the mesoderm similar to those seen 

 in figure 9. In figure 11 the blood-islands can be clearly seen in the gray bands, 

 that is, in the lumina of the vessels. 



In the living specimen the endothelium of a fully formed vessel has the appear- 

 ance of ground glass, as represented in figures 18 and 19, plate 4. The contrast 

 between the two stages, first, the interspaces between angioblasts, and second, the 

 interspaces between blood-vessels, is brought out by comparing figures 20 and 18, 

 plate 4, both of which are taken from living specimens and represent the actual 

 appearance as nearly as possible. The confusion in regard to determining the inter- 

 spaces from the lumen of the vessels (which is inevitable to one looking at such a 

 specimen for the first time) at a stage before there is any circulation is entirely 

 eliminated after the blood-cells move in response to the beat of the heart. In fixed 

 preparations the chance of confusion is not great. 



In an area in which many new blood-islands are forming, one can often find 

 unicellular blood-islands, two of which are shown in figure 18, plate 4. Again, such 

 an island is shown in a section in figure 21, plate 5. One specimen which was grow- 

 ing on the cover-slip had so many of these unicellular islands that there appeared 

 to be almost a duplication of the endothelium. In such specimens, fixed just during 

 the phase of cell division, I find the nuclear spindles placed perpendicular to the 

 wall of the vessel, so that the inner cell which is going to make the blood-island 

 projects directly into the lumen of the vessel at the very start. On the other hand, 

 the nuclear spindles usually lie in the plane of the endothelial wall, as can be seen 

 in any specimen in which the endothelium is dividing to increase the lining of the 

 vessels. 



These small unicellular blood-islands develop a granular cytoplasm, as can be 

 made out in figure 21, plate 5. Thus, from the evidence of the living specimen, the 

 small masses of cells, even the unicellular ones, are properly called blood-islands, 

 except for the fact that, strictly speaking, they are not islands at all, being always 

 attached to the walls of vessels. Not only are the islands yellow, but the cells are 

 uniformly yellow with hemoglobin, showing that all of them become erythroblasts. 

 This was pointed out by O. Van der Stricht (1892) in a study of the development of 

 the blood in the chick. He says (page 216) : 



"Des le premier stade, toutes les cellules sanguines pr&entent un aspect particulier. 

 Leur protoplasma est d'un jaune fence" plus compact que celui des elements voisins. 

 Chargees done d'une quantite d'h6moglobine plus ou moins considerable, elles out, des leur 

 origine, les caracteres du corpuscule rouge." 



