. The development of the chick; an introduction to embryology . thepatic vem. V. sc, Subcardinal vein. V. umb., Umbihcal vein. Large Intestine, Cloaca, and Anus. The cloaca of the adultis a large chamber opening to the exterior by the anus; it consistsof three divisions: the proctodseum or terminal chamber is capableof being by the sphincter muscle, the bursa Fabricii opensinto its dorsal wall, and it is separated by a strong circular fold ALIMENTARY TRACT AND ITS APPENDAGES 315 from the intermediate section of the cloaca or iirodaeum; this isa relatively short division of the cloaca w
. The development of the chick; an introduction to embryology . thepatic vem. V. sc, Subcardinal vein. V. umb., Umbihcal vein. Large Intestine, Cloaca, and Anus. The cloaca of the adultis a large chamber opening to the exterior by the anus; it consistsof three divisions: the proctodseum or terminal chamber is capableof being by the sphincter muscle, the bursa Fabricii opensinto its dorsal wall, and it is separated by a strong circular fold ALIMENTARY TRACT AND ITS APPENDAGES 315 from the intermediate section of the cloaca or iirodaeum; this isa relatively short division of the cloaca which receives the renaland reproductive ducts in its dorsal wall by two pairs of openings;it is bounded from the larger anterior division, coprodseum, bya rather low circular fold; the coprodaeum passes gradually, with-out a sharp line of division, into the rectum. The early embryological history of these parts has been con-sidered in the preceding chapters. The condition on the fourthday is shown in the accompanying figure (Fig. 183) representing a mMMMmmNhh. Fig. 183. — Median sagittal section of the hind end of a chick embryo on the fourth day of incubation. (After Gasser from Maurer.) All., Allantois. Am., Tail fold of amnion, cl. M., Cloacal mem-brane. CI., Cloaca. Nch., Notochord. n. T., Neural tube. R., Rec-tum. Y. S., Wall of yolk-sac. sagittal section of the hind end of the embryo. The cloaca is thelarge terminal cavity of the intestine, closed from the exteriorby the cloacal membrane, in which the entoderm of the floor ofthe cloaca is fused to the superficial ectoderm at the base of thetail. The line of fusion is a long, narrow median strip, extendingfrom just below the neck of the allantois to the hinder end of thecloaca. Leading out from the cloaca ventrally, in front of the 316 THE DEVELOPMENT OF THE CHICK cloacal membrane, is the neck of the allantois, and dorsal to this,the large intestine. Though not shown in the figure, it may benoted that the Wolffian ducts
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