118 THE DEVELOPMENT OF THE CHICK 
degrees nearer and nearer to the embryo, until, by the 7 or 8s 
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- 
tinuity 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 5s 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 
hemoglobin, 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 
