654 



Comparative Morphology of Chordates 



intestine 

 f\ I .Dorsal aorta 



_l / /Notochord 



>./ ,Neural tube 

 mm^ .Meso-metanephric ducts. 

 Cloaca 



Mullerian duct 



Urinary bladder 



Urinogenital sinus 



Perineum 



U 



Vagina 

 Vestibule 

 Pubic symphysis 



Urinary bladder 



Fio. 491. Diagrams illustrating the dividing of the mammalian embryonic 

 cloaca into a rectal and a urinogenital part. The diagrams represent sagittal sec- 

 tions of the caudal region of the embryo. The superficial ectoderm and the endo- 

 derm are shown in heavy lines; mesoderm is stippled. The amnion is removed 

 except a remnant attached to the embryo ventrally. 



(A) is a stage such as occurs in a pig embryo about 6 mm. long. This stage 

 shows the primary cloaca, and the cloacal membrane consisting of apposed ecto- 

 derm and endoderm. The cloacal aperture of the right mesonephric (Wolffian) 

 duct is shown. As the metanephros develops from the hind end of the mesonephric 

 duct, this aperture is for a time common to the mesonephros and the developing 

 metanephros. 



In (B) the proximal region of the allantoic outgrowth has become enlarged, 

 predicting the differentiation of a urinogenital sinus and urinary bladder. In (C) 

 the right Mullerian duct (oviduct) appears in close relation to the cloacal end of 

 the meso-metanephric duct. 



The arrows in (A), (B), and (C) refer to the region where the intestinal wall 

 bends ventrad to continue as the allantoic wall. As the cloacal space expands (by 

 growth of its walls), the bend ("uro-rectal fold") extends caudad (suggested by 

 the arrows) as seen in sagittal section. This intruding "fold," however, is not 

 confined to the mid-anterior region of the cloacal wall but continues back on each 

 side so that the cloacal space is constricted laterally as well as from in front. In 

 (C) the anterior edge of the "fold" has nearly reached the cloacal membrane. \ 

 little later (D) the constriction is completed and the cloacal membrane has become 

 divided into a dorsal anal membrane (a) and a ventral urinogenital membrane 

 (u), each destined to become perforated to form, respectively, anus and urino- 

 genital aperture. 



In (D) a constriction intervenes between urinogenital sinus and urinary bladder. 

 The mesonephric and metanephric ducts, by growth at their region of junction, 

 become separated. The metanephric duct (ureter) shifts forward onto the dorsal 



