130 EARLY EMBRYOLOGY OF THE CHICK 



until later in development, certain of its fundamental relation- 

 ships are established at this stage. 



The cloaca of an adult bird is the common chamber into 

 which the intestinal contents, the urine, and the products of 

 the reproductive organs are received for discharge. The first 

 appearance of the cloaca in the embryo as a dilated terminal 

 portion of the gut establishes at the outset the relations of 

 cloaca and intestine familiar in the adult. 



Although the urinary system is not at this stage developed 

 to conditions which resemble those in the adult the parts of it 

 which have been established are already definitely associated 

 with the cloaca. The proximal portion of the allantoic stalk 

 which is the homologue of the urinary bladder of mammals 

 opens directly into the cloaca (Fig. 43), When the urinary 

 system of the embryo is considered, we shall see that the ducts 

 which drain the developing excretory organs also open into 

 the cloacal region on either side of the allantoic stalk. 



There is at this stage but Httle indication of the for- 

 mation of the gonads. The relation of the sexual ducts 

 to the cloaca can be made out only by the study of older 

 embryos. 



The ProctodsBum and the Cloacal Membrane. — Indications 

 of the formation of the cloacal opening to the outside appear 

 during the fourth day of incubation. Its estabhshment is 

 accomphshed in much the same manner as the establishment 

 of the oral opening. A ventral out-pocketing of the hind-gut 

 arises just caudal to the point at which the allantoic stalk 

 opens into the cloaca (Fig. 43) . At the same time a depression 

 appears in the overlying ectoderm. The external depression 

 which grows in toward the gut pocket is known as the procto- 

 daeum. The double epithelial layer formed by the meeting of 

 gut entoderm with proctodeal ectoderm is the cloacal mem- 

 brane. The formation of the proctodaeum and the cloacal 

 membrane cleaily indicate the location of the future cloacal 

 opening although an open communication is not established 

 by the rupture of the cloacal membrane until considerably 

 later. The cloacal opening does not form at the extreme pos- 

 terior end of the hind-gut and there is, therefore, a post-anal 

 pocket of the hind-gut suggestive of the pre-oral pocket of the 

 fore-gut. 



