ee 
its o i fi 
, 
—— 
—— 
DEVELOPMENT OF CIRCULATING APPARATUS. 583 
758. During the progress of this change, another very im- 
portant one is taking place, whichis 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 
Fig. 317.—-Morz apvANcepD Em- Fig. 318.—Yorx-sac or Fowt's Ece, at 
BEYo oF FowL, much enlarged: 
the beginning of the third day of iucu- 
a, pellucid area; 4 4, dorsal bation : 
laminz 
; ¢¢, rudiments of ver- a, yolk; 6, embryo; ce, arteries of vascu- 
tebral arches; d, dilatation for lar area; d d, veins ; e e, terminal sinus. 
brain; e e, cephalic hood; f, 
chorda dorsalis, 
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, 2). Its wall is at 
