224 ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 



tion between the mesodermal masses that develop into angioblasts and the endoderm 

 there is no sign that the endoderm gives rise to angioblasts. 



A point which I wish to emphasize is that at the stage shown in figure 2, plate 

 1, the masses of mesoderm which are so conspicuous in the posterior zone of the 

 blastoderm are as yet without differentiation. They have been called blood-islands, 

 and it will be difficult to change a name with such a long history. I believe, however, 

 that it would be possible to show that these uniform masses of cells are destined, as 

 stated above, to differentiate into two different structures, neither one of which 

 are blood-islands; i. e., two layers of cells placed along the dorsal border of the 

 masses, which are the forerunners of the mesoderm which will border the coelom, 

 and ventral clumps of angioblasts. Moreover, the angioblasts must first make 

 blood-vessels in the chick before any clumps of cells form hemoglobin and become 

 the ancestors of red blood-corpuscles. There is no differentiation in most of the 

 primitive masses of mesoderm before the somites develop, as indicated in figure 14, 

 plate 4, where the dark spots along the ventral border of the mesoderm, seen espe- 

 cially well on the left side, are merely cells in division. In this section the ectoderm 

 is readily distinguishable, for the reason that its cells form a definite row with many 

 nuclei equally spaced. The mesodermal masses have larger, irregularly placed 

 nuclei, and more vacuoles and more droplets of yolk in the cytoplasm. 



The next stage in the development of the coelom is shown in plate 1, figure 

 3, a blastoderm of 2 somites. This is from a chick which was grown for 3| hours 

 on a cover-slip. When first taken from the egg it had many endodermal blis- 

 ters, but they had all disappeared before the specimen was fixed. It showed a 

 primitive streak and a head-fold but no somites. While it was growing on the 

 cover-slip the first cleft, which determines the first and second somites, appeared, 

 and when fixed, the first and second somites were clear. They are still so delicate 

 as to be seen but faintly in the photograph. The latter discloses an interesting 

 point in connection with the primitive streak, namely, a transverse wrinkling 

 of the streak which is a constant phenomenon in the blastoderms of early stages 

 when grown on a cover-slip. It indicates, I think, that the streak is a zone of 

 active growth in length, so that the fixing of the margins of the blastoderm to the 

 cover-slip consequently produces a wrinkling of the tissues at that point. The 

 same wrinkling of the primitive streak is shown in section in figure 14, plate 4. The 

 specimen shown in figure 3, plate 1, was growing vigorously when it was fixed, for 

 the entire endoderm is caught in the phase of nuclear division. There is a defect in 

 a portion of the mesoderm of the posterior part of the area opaca which is not an 

 uncommon abnormality in these chicks that have been grown on a cover-slip. The 

 defect apparent in the mesoderm on the right side of the photograph is not a real 

 one, but a zone where the cells have reacted less to hematoxyhn. The specimen was 

 chosen in spite of these defects because it shows so well the formation of the exocoe- 

 lom in the area opaca. 



Throughout the area opaca the mesoderm has formed a network of hollow 

 vesicles so closely packed together that the interspaces are mere lines of cells. In 

 this particular specimen almost all of the primitive mesoderm, as seen in figure 2, 



