ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 249 



in connection with the question of the ultimate fate of these cells. All of the cells 

 in these islands in the chick of 2 days of incubation become erythroblasts. All of 

 the observers of this process in mammals agree in considering these masses to be 

 made of primitive hemoblasts or mesamceboid cells, in the sense of Minot, or 

 primitive lymphocytes in the sense of Maximow; that is, capable of producing both 

 red cells and lymphocytes. That they produce erythrocytes is generally conceded, 

 and the evidence that they also produce white corpuscles, that is lymphocytes, is 

 that in many of them none of the signs of hemoglobin can be made out. 



The next phases in the development of the vascular system are brought out 

 in two photographs from chicks with 11 somites, figures 8 and 9, plate 2. In both 

 of these figures there is a massive plexus of solid angioblasts in the posterior part 

 of the area pellucida, which is characteristic of this stage. The blastoderm in figure 

 8, plate 2, had 10 somites when the specimen was taken from the shell, and the heart 

 was just twitching. It shows a great advance over the stage of 7 somites, for the 

 myocardium has fused across the mid-line, the heart has become a vessel, and con- 

 trary to the usual form has curved to the left side of the embryo (right side of the 

 photograph). The specimen was fixed during the phase of cell-division for the 

 angioblasts, as is shown in figure 24, plate 5. In this specimen the two sides are not 

 symmetrical in regard to the area opaca. On the right side it will be noticed that 

 there is not a very sharp contrast between the outer and inner rims of the posterior 

 part of the area opaca. This is because the outer rim is full of blood-cells and the 

 inner rim is still made of solid angioblasts. On the left side of the figure there is a 

 part of the inner rim where the angioblasts have become vessels and almost all of 

 the angioblasts have liquefied, the small dark spots representing tiny blood-islands. 

 Figure 9, plate 2, is a photograph of a chick of the same stage, in which all of the 

 angioblasts of the inner rim of the area opaca have liquefied. This specimen also 

 had 10 somites when taken from the shell, and the heart was just twitching; it 

 stopped beating very soon, but the cells continued to live, as they were in process 

 of division when the blastoderm was fixed. The specimen has some abnormalities 

 of the brain and several defects in the mesoderm, but it is very striking for the 

 mass of angioblasts in the posterior part of the area pellucida and for the almost 

 complete liquefaction of the angioblasts of the inner rim of the area opaca. From 

 the study of these living forms, I believe this very complete liquefaction of the 

 angioblasts is the rule rather than the exception that it to say, it is the rule in the 

 formation of the vessels in these early stages. 



To return to figure 8, plate 2, vessels have formed throughout the area pellucida, 

 except in the posterior part, but they are so delicate as not to show in the photograph. 

 They are empty except for a few free blood-cells which have formed in the area 

 since there has been no circulation at this stage. These corpuscles may oscillate back 

 and forth with the beating of the heart, but are not moved through the heart until 

 about the stage of 16 or 17 somites, when the circulation begins. A study of this 

 figure will show that if a transverse section be taken through the posterior part of 

 such an embryo one can see all the processes in the formation of blood-vessels and of 



