DEVELOPMENT OF 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 



Fig. 317. MORE ADVANCED EM- 

 BRYO OF FOWL, much enlarged : 



a, pellucid area; b b, dorsal 

 laminae ; c c, rudiments of ver- 

 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 e, 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. 32Q, h). Its wall is at 



