222 THE DEVELOPMENT OF THE CHICK 



the allantois (Fig. 130 B) ; at the end of the eighth day the alhm- 

 tois has covered half of the yolk-sac (Fig. 130 C). At the end 

 of the ninth day, the formation of the albumen-sac is begun 

 (Fig. 130 D). At the end of the eleventh day, the albumen-sac 

 is practically closed at the lower pole. On the twelfth day, the 

 albumen-sac is closed, and on the sixteenth day the contents 

 are practically entirely absorbed. 



Blood-supply of the Allantois. There are two allantoic (um- 

 bilical) arteries and one allantoic vein. (See Chap. XII.) Both 

 arteries persist throughout the period of incubation, but the 

 left is much the better developed. It passes out along the stalk of 

 the allantois to the inner wall of the allantoic sac, where it divides 

 in two strong branches, one running cephalad and the other caudad 

 to the margins of the sac where they pass over to the outer wall; 

 The allantoic vein runs in the inner wall and passes over to the 

 outer wall near the sero-amniotic connection. Both arteries and 

 veins inhibit the expansion of the allantoic sac where they sur- 

 round the margin; but the vein has by far the greatest effect, 

 as its action is supplemented by the sero-amniotic connection. 

 Thus indentations, gradually growing deeper, are established 

 along the margins of the allantoic sac, and the outgrowth of the 

 latter on each side of the indentations produce overlapping lobes 

 (Figs. 130 C and D). 



The arrangement of the smaller vessels and capillaries is 

 very different in the outer and inner walls. In the outer wall 

 the arteries and veins branch and interdigitate in the deeper 

 portions of the mesoblast, and end in an extraordinarily fine- 

 meshed capillary netw^ork situated immediately beneath the thin 

 ectoderm. "The capillaries form such narrow meshes, and have 

 relatively so wide a lumen, that they can be compared only with 

 those of the lungs of higher animals, and of the choroidea of 

 the eye; indeed, instead of describing it as a vascular network 

 embedded in tissue, one could as well describe it as a great 

 blood-sinus interrupted by strands of tissue" (FiiUeborn.) This 

 capillary network of the outer wall constitutes the respiratory 

 area of the allantois. At the margins it passes gradually into 

 the incomparably wider meshed capillary network of the inner 

 wall. An extensive system of lymphatics is developed, l^oth 

 in the outer and inner walls of the allantois, accompanying all 

 the blood-vessels, even to their ultimate terminations. 



