MA Kl.ol'MKM 'F ; ' BLOOD. 



\Vc must thrivfor.' now di>tingui>h in tin- opa^it'' aiv:i ( Plate I., 

 iiiT. L\ page 213) two rin- like areas, t/.,> r,,.<cular art'" -? the 



yolk-area (dh), ar< fl and ar>-a Since, moreover, 



sx, 



Fig 117. Diagram of the vascular system of the yolk-sac at the end of the third day of 

 incubation, after BALKOUR. 



The whole blastoderm has been removed from the egg and is represented as seen from below. 

 Therefore what is really on the left appears on the right, and vice vtrsd. The part of the 

 area opaca in which the fine vascular network has been formed is sharply limited at the 

 periphery by the sinus terminalis, and represent* the vascular area; outeide of it lu-s th,- 

 yolk-area. The immediate vicinity of the embryo is destitute of a vascular network, and is 

 designated now, as at an earlier stage, by the name area pellucida. 



;/ Heart; A A, aortic arches; Ao, dorsal aorta, L.Of.A, left, R.Of.A, right vitelline artery; 

 S.T, sinus terminalis ; L.Of, left, R.Of, right vitellin? vein ; X. J', sinus venosus ; D.C, ductus 

 t'uvieri ; s.c,t.}', superior, V.Ca, inferior cardinal vein. The veins are drawn in outline, 

 the arteries in solid Mark. 



tho area pelhu ida i> >till recognisable, being traversed by only a few 

 chief trunks of blood-vessels leading to tbe embryo, tbe body of the 

 embryo is enclosed altogether by three zones or ;in.i-of the extra - 

 embryonic part of the germ-layers. 



Up to the present \ve have pursued the formation of blood in the 

 opaque area. But how do the vessels in the body of the embryo 



