238 ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 



The young islands, as seen in figure 18, plate 4, have a smooth contour, even 

 a very definite border. Neither in the living nor in the fixed specimen can one see 

 any cell outlines within the mass. Nevertheless each nucleus has its own zone of 

 cytoplasm which closes in about it when the cells divide. Just as all of the 

 angioblasts divide at the same time (fig. 24, plate 5), so all of the cells in all of 

 the islands of a given area become highly refractive at the same time. This period 

 of high refractivity lasts about an hour or more, and then the observer sees one 

 nucleus after another pass into the metaphase with the chromosomes on the spindle. 

 This is shown in the small drawing (fig. 18); the specimen was fixed just as the 

 record was made on the drawing of the division of the nuclei, and the nuclei were 

 found in the stained specimen in the metaphase. A photograph of this specimen 

 is shown in figure 12, plate 3. Throughout their early stages the contours of the 

 blood-islands are all round and smooth, except for a very interesting phenomenon 

 which is often to be observed, namely, that delicate sprouts of cytoplasm, exactly 

 like those by which the original angioblasts fuse to make a plexus, are put out from 

 the islands. These sprouts creep along the inner wall of the vessels and attach 

 themselves like guy ropes to the wall. They show a very marked tendency to join 

 other neighboring blood-islands, and I have often seen sprouts from two islands 

 (like the small island on the left of figure 18, plate 4, and the nearby unicellular 

 island) join by sprouts which meet half way between the masses. In this way com- 

 pound islands are formed, cells filling in along the guy ropes until the whole mass 

 becomes a single island. At other times these new sprouts simply seem to serve 

 as extra guy ropes by which the islands are more firmly anchored to the wall. In 

 the living chick the presence of a circulation in a given area does not interfere with 

 the development of the blood-islands, though it is clear that the venous zone of the 

 area vasculosa is less active in the production of new islands than the more pos- 

 terior arterial zone when the circulation is later established. I have not noticed 

 that a specimen showing a very marked tendency to form islands is one in which 

 there is vigorous circulation in the area in question. The large compound islands 

 often completely shut off the circulation in a given channel, and in the living speci- 

 men I have observed that blood-cells which have been circulating get caught by 

 the bridges crossing the lumen and become incorporated into a growing island. 



This property of the blood-islands to send out the guy ropes by which they 

 become anchored in many places to the neighboring wall is duplicated by the 

 Kupffer cells in the liver. In sections of the liver of a rabbit that has received 

 repeated doses of trypan-blue one can get an exact duplication of the picture of 

 numerous unicellular blood-islands seen in the young blastoderm of the chick. In 

 examining sections of the liver which have been stained with carmine or any 

 nuclear dye after repeated doses of trypan-blue, one will often find the nucleus of 

 the parent endothelial cell behind a Kupffer cell exactly as that shown in figure 21, 

 plate 5, in the unicellular island labeled B. i., from a chick of 17 somites. Thus it 

 seems to me clear that Kupffer cells do not make an endothelial lining of the capil- 

 laries of the liver but constitute another generation of cells from the endothelial 

 wall of these capillaries, exactly like the blood-islands in the embryo chick; and that 



