S6 THE DEVELOPMENT OF THE CHICK 
with instruments of greater precision one cannot feel certain of 
the results of such experiments. 
Ill. Tue MrsopERM OF THE Opaque AREA 
We have seen that the mesoderm arises from the sides of the 
head-process and the primitive streak, and grows out between 
the ectoderm and the entoderm to the margin of the pellucid 
area; it then begins to overlap the opaque area at first behind, 
later at the sides, appearing between the ectoderm and the germ- 
wall. Figs. 44 A, B, C, and 45 illustrate its peripheral extension; 
at first it spreads most rapidly behind the embryo, but soon ex- 
tends with equal speed opposite the primitive streak, and thus 
a considerable portion of the area opaca becomes three-layered, 
consisting of ectoderm, mesoderm, and germ-wall (Figs. 40 C 
and 41). The contour of the anterior margin of the mesoderm 
it as first rounded, convex anteriorly (Figs. 44 A and B). Then 
the antero-lateral angles of the mesoblast begin to extend forward 
so that the anterior boundary becomes concave (Fig. 44 C); the 
lateral horns thus established continue to grow forward and 
ultimately meet in front of the head (Mig. 45); they thus bound a 
mesoblast-free area in front of and beneath the head, known 
as the proamnion, into which the mesoderm does not penetrate 
until a relatively late stage of development. 
Blood-islands (Figs. 44 C and 45) develop early in the three- 
layered part of the opaque area; appearing first behind the em- 
bryo, they rapidly differentiate forward opposite the sides of 
the embryo and follow the expansion of the mesoblast. This 
three-layered portion of the opaque area is known as the vascular 
area (area vasculosa) after the appearance of the blood-islands. 
It soon acquires a very definite peripheral boundary by the forma- 
tion of the vena (sinus) terminalis at its margin (Fig. 45). The 
two-layered peripheral portion of the opaque area is known as 
the vitelline area (area vitellina), and here again we distinguish 
two zones, an outer including the zone of junction,and an inner 
one (Figs. 32, 33). 
The first blood-islands are masses of cells lying on the germ- 
wall behind the embryo; the first blood-cells (erythrocytes) and 
blood-vessels arise from them, hence their name. Soon after 
their origin the blood-islands appear red owing to the formation 
of hemoglobin. Between the blood-islands and the eetoderm 
