. The development of the chick; an introduction to embryology. Birds -- Embryology. 226 THE DEVELOPMENT OF THE CHICK sides may be approximate!}^ in contact, and the cavity of the volk-sac is thus broken up into numerous connecting compart- ments filled with yolk. The outer wall of the yolk-sac is smooth and not involved in the folds. The beginning of the folds of the volk-sac mav be found at the time of appearance of the vascular area of the blastoderm, and they develop pari passu, with the vessels of the yolk-sac (Fig. 131). Fig. 131 shows the appearance of the folds at the stage of twelve so
. The development of the chick; an introduction to embryology. Birds -- Embryology. 226 THE DEVELOPMENT OF THE CHICK sides may be approximate!}^ in contact, and the cavity of the volk-sac is thus broken up into numerous connecting compart- ments filled with yolk. The outer wall of the yolk-sac is smooth and not involved in the folds. The beginning of the folds of the volk-sac mav be found at the time of appearance of the vascular area of the blastoderm, and they develop pari passu, with the vessels of the yolk-sac (Fig. 131). Fig. 131 shows the appearance of the folds at the stage of twelve somites. It is a view of the blastoderm from '^ Fig. 131. — Septa of the yolk-sac as seen on the lower surface of the blastoderm at the stage of 12 s. (After Hans Virchow.) m. R., Marginal ridge of entoderm overly- ing the sinus terminalis. drawn as an opaque object, and it shows the incipient folds of the yolk-sac in an arrangement that corresponds roughly, but not accuratelv, with that of the blood-islands, which lie in large part in the bases of the folds. The site of the vena terminalis is marked bv a circular fold of the entoderm. The folds of the yolk-sac thus coincide in their distribution Avith the vascular area and are so limited at all times, being absent in the vitelline area. There is thus a close connection between the vitelline blood-. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Lillie, Frank Rattray, 1870-1947. New York, Holt
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