ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 227 



where there are more angioblasts to be seen against the background of the 

 ccelom. This is shown on plate 1 , figure 5, and plate 2, figure 6, and also in a drawing 

 from a small part of the area pellucida of one of them, figure 27, plate 6. Thus 

 there is a stage when the ccelom appears as a plexus of undifferentiated mesoderm 

 dorsal to the plexus of angioblasts. 



The plexus of the exoccelom, as shown in figure 27, plate 6, is a delicate network 

 of protoplasm, with nuclei in the nodes and vacuoles and delicate fibrils in the cyto- 

 plasm. This network looks not at all unlike a typical mesenchyme except that it is 

 always in definite layers. Here and there are large interspaces which are entirely 

 devoid of mesoderm. Against this delicate network can be seen the massive bands 

 of angioblasts, still connected in places with the parent mesoderm beneath by 

 bands of cells, and with each other by the sprouts so characteristic of angioblasts. 

 The difference between these sharp, definite sprouts (representing the method 

 by which the angioblasts join each other) and the delicate fibrils in the mesh of the 

 mesoderm is well shown in the drawing. Throughout the stages of from 3 to 8 

 somites these networks must be studied together. The existence of the two net- 

 works, that of the ccelom and the more ventral plexus of angioblasts, was recognized 

 by His (1868). 



It is not my purpose to follow any further in this paper the development of the 

 coclom, because, as the chick grows older, it becomes less and less a source of con- 

 fusion in connection with the study of the vascular system. Up to the stage of 5 to 

 7 somites one must be. thoroughly familiar with the different appearances of the 

 ccelom in the different parts of the area vasculosa, but as time goes on the ventral 

 space occupied by the vascular layer grows wider and the two structures thus cease 

 to be so nearly at the same focussing level; thus the difficulty in keeping them quite 

 distinct practically disappears. In general, however, in studying the living chick 

 there are three different zones of mesoderm in the area pellucida against which the 

 angioblasts must be studied; first, the area of large vesicles which, by fusion into a 

 single cavity, give rise to the amnio-cardiac cavities; second, the fine network of 

 mesoderm of the middle and posterior zones of the area pellucida; and third, the 

 axial zone of the somites and the dense, undifferentiated mesoderm posterior to 

 them. This posterior, axial zone of massive mesoderm is so dense that only an 

 occasional specimen will show the angioblasts with any great clearness against this 

 background. It is, however, an important area for the study of the aorta. Fortu- 

 nately, the angioblasts of the axial zone opposite the interspaces between the 

 somites are especially clear in the living chick, while the angioblasts of the posterior 

 zone over the undifferentiated mesoderm, opposite which the posterior end of the 

 aorta develops, can also be seen much more clearly in the living specimen in which 

 the cells are dividing. 



DIFFERENTIATION OF ANGIOBLASTS FROM MESODERM. 



At a given stage in the development of the mesoderm of the chick, 

 certain cells differentiate from the mesoderm to become the forerunners of 

 the vascular system. The sequence of this differentiation will be taken up later, 



