118 THE DEVELOPMENT OF THE CHICK 



degrees nearer and nearer to the embryo, until, by the 7 or 8 s 

 stage, blood-vessels begin to appear in the embryo itself. It is 

 important to note that the order of appearance of the vascular 

 primordia is first in the area opaca in the order previously de- 

 scribed, then in the pellucid area and finally in the embryo itself. 

 Moreover, the parts appearing later are, usually at least, in con- 

 tinuitv with those first formed. 



Before discussing the way in which the blood-vessels arise 

 in the pellucid area and in the embryo, we should consider the 

 first differentiation within the original, or peripheral, blood- 

 islands. Between the 3 and 5 s stage it may be noticed in 

 sections that vacuoles are forming within the peripheral blood- 

 islands near the entodermal surface. The expansion of these 

 vacuoles carries the peripheral layer of cells away from the main 

 mass of cells composing the blood-islands, and by degrees the 

 process is carried completely around the blood-island, so that 

 the peripheral layer becomes entirely separated from the central 

 mass and encloses it (See Fig. 68 C). The enclosing cells become 

 flattened during this process to form an endothelium; inasmuch 

 as the blood-islands are not separate, but anastomose to form a 

 network, the process results in the formation of a network of 

 endothelial tubes enclosing cell-masses. Thus arise the first 

 blood-vessels. The enclosed masses of cells rapidly acquire 

 haemoglobin, become separated from one another, and form 

 blood-cells. 



There is a great difference in the relative amounts of blood- 

 cells formed in different regions. Thus in the anterior part of 

 the opaque area and in the pellucid area the original blood- 

 islands are relatively small (Figs. 44 and 45), and furnish material 

 sufficient only for the formation of the blood-vessels. On the 

 other hand, in the peripheral part of the vascular area, especially 

 towards its posterior end, the largest masses of blood-cells are 

 found; and these conditions grade into one another. In other 

 words, the formation of blood-cells is restricted at this time to 

 the opaque area, and is most abundant posteriorly. In the 

 pellucid area only empty blood-vessels are formed. Similarly 

 the blood-vessels of the embryo itself are at first empty; they 

 become filled secondarily from the opaque area when circulation 

 begins. 



The appearance of blood-vessels within the pellucid area 



