. The development of the chick; an introduction to embryology . iciiiaii .sagittalsection of an 8-day , Allantois. An., Anus. B. F., Bursa Fahricii. caud. .\., (audalartery. Int., Intestine. Nch., Notochonl. p. P., Perianal papilla. Rect.,Rectinn. I^rVI., Urodannn. During the twelfth and thirteenth days, the vacuoles in theupper ])art of the large intestine flow together and re-estal) ca\itv. but the lower end still remains closed by a solid plugof cells; immediately anterior to the latter the large intestine isdilated, and this apparently corresponds to the coi)r()dieuni of
. The development of the chick; an introduction to embryology . iciiiaii .sagittalsection of an 8-day , Allantois. An., Anus. B. F., Bursa Fahricii. caud. .\., (audalartery. Int., Intestine. Nch., Notochonl. p. P., Perianal papilla. Rect.,Rectinn. I^rVI., Urodannn. During the twelfth and thirteenth days, the vacuoles in theupper ])art of the large intestine flow together and re-estal) ca\itv. but the lower end still remains closed by a solid plugof cells; immediately anterior to the latter the large intestine isdilated, and this apparently corresponds to the coi)r()dieuni of ALIMENTARY TRACT AND ITS APPENDAGES 319 the adult cloaca. Even on the seventeenth day the large intes-tine appears to be still closed at its lower end, and the urodaealmembrane still persists as a plug of vacuolated cells. (Gasser.)Both plugs must, however, disappear soon after. It would thus appear that the urodseum only of the adultcloaca corresponds to the embryonic cloaca; the proctodseum iscertainly derived from an ectodermal pit, and it is probable that. Fig. 186. — Chick embryo of 11 days, sagittal sectionthrough the region of the cloaca. Reconstructed fromseveral sections. (After Minot.)All., Ascending limb of the aUantois. All., Descend-ing limb of the allantois. An., Anal invagination. An. pi.,Urodeal memlirane. Art., Umbilical artery. B. F.,Bursa Pabricii. b. f., Duct of the bursa. Clo., , Ectoderm. Ent., Entoderm of the rectum. Ly.,Nodules of crowded cells, probably primordia of lym-phoid structures in the wall of the large intestine. W. D.,Wolffian duct. the coproda?um represents the enlarged lower extremity of theembryonic large intestine. The bursa Fabricii is an entodermalstructure derived from the posterior portion of the embryonic;cloaca. IV. The Development of the Liver and Pa\cre.\s The Liver. The anterior and posterior liver diverticula, de-scribed in Chapt(>r VI, constitute the rudiments from which tlie ,S20 THE DEVELOPMENT OF THE CHICK sul)s
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