ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 243 



the large vesicles of the coelorn (which do not show in the photograph) and masses 

 of solid angioblasts. There has been no liquefaction of the angioblasts. The area 

 pellucida has three or four masses of angioblasts in the zone of the amnio-cardiac 

 vesicles, all of them lying opposite the borders of the vesicles of the coelom. Oppo- 

 site the venous end of the heart in the mid-zone the angioblasts are too delicate to 

 show at this magnification, but they are present, and some of them have even 

 formed tiny vessels. In the posterior zone of the area pellucida the angioblasts are 

 more massive and have already been shown in a drawing to illustrate their contrast 

 to the coelom (fig. 27, plate 6). In the axial line between the mycardium and the 

 edge of the head-fold are a few tiny angioblasts, too small to be seen, which are the 

 forerunners of the endocardium. These are shown in figure 1, in the text, at the 

 stage of 6 somites. Between the myotomes a higher magnification shows the first 

 angioblasts of the aorta. 



In contrast to this specimen of 5 somites is one in figure 6, plate 2. In this 

 blastoderm it is clear that the area opaca has now become divided into two zones, 

 an outer and an inner. The two leaders on the left hand indicate the width of the 

 zones. Over the whole outer zone the angioblasts have become transformed into 

 vessels by the liquefaction of most of the angioblasts. This is illustrated in figure 

 22, plate 5, which is a drawing of the area outlined in figure 6, plate 2. In this 

 drawing it will be seen that there are vessels on the left side forming a blood-island 

 while on the right similar masses of cells lead directly over into a plexus of solid 

 angioblasts which have not yet become vessels. The large blood-islands can be 

 readily compared with the same mass in the photograph. In the specimen it can 

 be noticed that this blood-island is perceptibhy darker than the angioblasts just 

 internal to it ; this is because the island has enough hemoglobin in its cells to be 

 made out in the counterstain. By a study of the photograph it can be seen that a 

 considerable amount of the original mass of angioblasts has been used up in the 

 formation of the plasma in the outer zone; perhaps less than half of the original 

 mass has remained in the form of blood-islands. 



There is undoubtedly a considerable variation in regard to the time at which 

 the angioblasts of the outer rim develop into vessels; in some of my specimens of 

 6 somites the change has not been made and Ruckert (1906) states that vessels 

 with lumina are to be found only in chicks with about 7 somites, and not anywhere 

 in the area vasculosa at the stage of 6 somites (p. 1224, fig. 890). In my specimen 

 of 5 somites, on the other hand, the transformation of the entire mass of angio- 

 blasts into vessels in the outer rim of the area opaca has taken place, and I judge 

 that this is about the earliest stage in which one is likely to find the condition. As 

 has been said, in this outer zone it is obvious that the amount of blood-islands left 

 is certainly not more than about half of the original mass of angioblasts, indicating 

 that a considerable amount of the original mass of angioblasts has been used up in 

 the formation of endothelium and plasma. I believe that this is the usual condi- 

 tion in the formation of the early vessels. The vessels will soon fill up almost com- 

 pletely with new masses of blood-cells, but wherever I have observed the actual 



