. The development of the chick; an introduction to embryology . point of opening into the highest point of theproctodseum. Vacuolization of the tissue between the cloacalmembrane and the urodasum indicates its subsequent dis-appearance. At eleven days (Fig. 186) the general arrangement is essen-tially the same, but there are important differences in bursa Fabricii has now become a long-stalked sac, openinginto the proctodseum at the level of the urodseal latter is still quite a thick plate, but the vacuoles in it fore- 318 THE DEVELOPMENT OF THE CHICK shadow its final r
. The development of the chick; an introduction to embryology . point of opening into the highest point of theproctodseum. Vacuolization of the tissue between the cloacalmembrane and the urodasum indicates its subsequent dis-appearance. At eleven days (Fig. 186) the general arrangement is essen-tially the same, but there are important differences in bursa Fabricii has now become a long-stalked sac, openinginto the proctodseum at the level of the urodseal latter is still quite a thick plate, but the vacuoles in it fore- 318 THE DEVELOPMENT OF THE CHICK shadow its final rupture. The lower end of the large intestineis perfectly solid, and higher up, somewhat vacuolated. (Thesolid stage begins on the seventh day.) The urinogenital ductsopen into the urodseum above the solid end of the large will be seen, therefore, that the urodseum is transformed intoa passageway between the urinogenital ducts and the allantois,being closed anteriorly by the solid large intestine and posteriorlyby the urodseal (cloacal) Fig. 185. — Photograph of the region of the cloaca in a median sagittal section of an 8-day chick. All., Allantois. An., Anus. B. F., Bursa Fabricii. caud. A., Caudalartery. Int., Intestine. Nch., Notochord. p. P., Perianal papilla. Rect.,Rectum. Urd., Urodseum. During the twelfth and thirteenth days, the vacuoles in theupper part of the large intestine flow together and re-establishthe cavity, 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 coprodaeum of ALIMENTARY TRACT AXD 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 urodsealmembrane still persists as a plug of vacuolated cells. (Gasser.)Both plugs must, however, disappear soon after. It would thus appear that the urodgeum only of the adultcloaca correspond
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