ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 223 



different areas of the ccelom, as they can be made out in the developing chick, with 

 reference to the origin of the blood-vessels. At certain stages the exoccelom in the 

 living chick simulates the blood-vessels, although formed by an entirely different 

 process. 



The specimen shown in figure 2, plate 1, is in what may be termed the second 

 stage in the development of the mesoderm of the chick. As is well known, in early 

 chick blastoderms of the first day of incubation there is a stage in which the primi- 

 tive streak has formed, but before the appearance of a head-fold, when the mesoderm 

 is spread out uniformly through the area pellucida and the area opaca except in the 

 pro-amnion. At the stage shown in figure 2, plate 1, it is evident that the mesoderm 

 is more dense opposite the posterior half of the area opaca, and that the outer rim of 

 the area opaca is much mottled. These patches of cells would, of course, be uni- 

 versally termed the blood-islands of Wolff, but I wish to question the use of the 

 term blood-islands for them. In sections the mottling is seen to be due to isolated 

 patches of mesoderm closely attached to the ectoderm (plate 4, fig. 14). Moreover, 

 it can be noted that though the dorsal margin of the ectoderm has a smooth 

 contour, the ventral border has many filamentous processes pointing toward the 

 endoderm, and that here and there one sees a nucleus in this ragged border of the 

 ectodermal cells which might be interpreted as the beginning of a new group of 

 mesodermal cells from ectoderm. 



To the evidence that mesoderm may arise from ectoderm in situ, along the edge 

 of the area opaca, may be added that in later stages, after the sinus terminalis has 

 formed, one can still find quite isolated masses of new mesodermal cells lateral to 

 the sinus. Vialleton (1892) called attention to these small masses of cells, attached 

 to the ectoderm, which are to be found entirely outside the area vasculosa in a 

 chick of 11 somites, and interpreted them as the blood-islands of Wolff. During 

 the next year O. Van der Stricht published a note in which he questioned the con- 

 clusions of Vialleton, on the ground that these masses of cells had none of the char- 

 acteristics by which they could be interpreted as blood-islands. This view coin- 

 cides with my own interpretation of them i. e., that, like the primitive mesoderm 

 within the area vasculosa, they constitute undifferentiated masses of cells and thus 

 are not to be identified as blood-islands; and that they are destined to form two 

 different tissues, the two layers of cells which form the exoccelom, and the clumps 

 of angioblasts. They are of especial interest as bearing on the question of the origin 

 of the mesoderm. 



Thus the evidence seems to me to point toward the view that while the primi- 

 tive mesoderm of the chick and the mass of mesoderm of the area vasculosa arise 

 at the border of the primitive streak or in the axis of the embryo for the head- 

 mesoderm, there may be a continual, considerable increase in the mesoderm of the 

 area opaca by new cells which are differentiated in situ from the ectoderm along the 

 border of the area vasculosa. This is in accord with the view of His and O. Van 

 der Stricht, as opposed to that of v. Kolliker, Hertwig, and Rabl. As was noted 

 by 0. Van der Stricht (1895, page 184), no matter how intimate may seem the rela- 



