DEVELOPMENT OP CIRCULATING APPARATUS. 
583 
758. During the progress of this change, another very im¬ 
portant one is taking place, which is destined for the nourish¬ 
ment of the embryo during its further development. This 
is the formation of vessels in the substance of the germinal 
membrane ; which vessels serve to take up the nourishment 
supplied by the yolk, and to convey it through the tissues 
of the embryo. The space over which these vessels spread 
tebral arches; d, dilatation for 
brain; e e, cephalic hood; /, 
chorda dorsalis. 
Fig. 318.— Yolk-bag of Fowl’s Egg, at 
the beginning of the third day of incu¬ 
bation : 
a, yolk; 6, embryo; c c, arteries of vascu¬ 
lar area; d d , veins ; e s, terminal sinus. 
themselves, is called the Vascular Area ; it makes its ap¬ 
pearance during the second day of incubation in the Fowl's egg 
(fig. 316, c), and soon spreads itself over the surface of the yolk 
(figs. 318, 319). Islets or points of a dark colour first appear 
in it; these unite in rows ; and at last continuous vessels are 
formed. The heart makes its appearance at the twenty-seventh 
hour of incubation, as a simple dilatation of the trunk into 
which the blood-vessels unite (fig. 320, h). Its wall is at 
