222 THE DEVELOPMENT OF THE CHICK 
the allantois (Fig. 130 B); at the end of the eighth day the allan- 
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-sae 
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 arteries 
and one allantoic vein. (See Chap. XII). Both arteries persist 
throughout the period of incubation, but the left is much the 
best 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. 1380 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 network 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’? (Fiilleborn.) This 
‘apillary 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, both 
in the outer and inner walls of the allantois, accompanying all 
the blood-vessels, even to their ultimate terminations. 
