788 THE EXCRETORY AND REPRODUCTIVE SYSTEMS 



into the wall of the caudal bladder region, and a considerable amount of 

 mesoderm is contributed to the entodermal lining of the developing bladder. 

 This mesodermal area presumably forms a part of the lining tissue of the 

 bladder (fig. 349 A, B). The metanephric duct or ureter, in the meantime, 

 shifts its position anteriad and becomes united with the dorso-posterior por- 

 tion of the bladder, while the point of entrance of the mesonephric duct mi- 

 grates posteriad to empty into the anterior end of the dorsal region of the 

 urogenital sinus (figs. 348F, G; 349A, B). 



In turtles and in some lizards, the adult relationships of the urinary bladder 

 and rectum are established in a somewhat similar manner to that of the 

 mammals, although the caudal migration of the cloacal septum is not ex- 

 tensive. Also, the cloaca is retained. 



The urinary bladder (or bladders) of some teleost and ganoid fishes arise 

 as swellings and evaginations of the caudal ends of the mesonephric ducts 

 (fig. 347B). A distinct urinary bladder is absent in elasmobranch fishes and 

 in birds, but is present in amphibia as a ventral diverticulum of the cloaca. 



c. Cloaca 



A cloaca into which open the urogenital ducts and the intestine is a common 

 basic condition of the vertebrate embryo. It is retained in the definitive or 

 adult body form of elasmobranch fishes and to a considerable extent in dipnoan 

 fishes. It is present also in the adults of amphibia, reptiles, birds (fig. 347C), 

 and prototherian mammals. A cloaca is dispensed with in the adult stage of 

 teleost (fig. 347B) and ganoid fishes, and also in the adult stage of higher 

 mammals (fig. 349A-D). 



C. Development of the Reproductive System 



The general features of the adult condition of the reproductive system are 

 described in Chapters 1 and 2. For most vertebrates, the reproductive system 

 consists of the reproductive glands, the ovaries or testes, and the genital ducts. 



FiG. 349. Differentiation of the caudal urogenital structures in the human embryo. 

 (A) Later stage in differentiation of the cloaca; the rectal area is being separated from 

 the ventraliy placed urogenital sinus by the cloacal (urorectal) membrane. Condition 

 of sixth week (about 12 mm.) embryo. (B) Rectal and urogenital areas completely 

 separated. Miillerian and mesonephric ducts present. Metanephric duct has moved for- 

 ward into the posterodorsal area of the developing bladder. The Miillerian ducts have 

 fused at their caudal ends to form the uterovaginal rudiment. This condition is present 

 at about 8 weeks. (C) Male fetus of about 5 months. Testis beginning to pass into 

 developing scrotal sac. (See also fig. 3.) (D) Female fetus of about 5 months. 

 (E to K) Stages in development of external genitalia. (E) Indifferent condition (about 

 7 weeks). (F) Male about tenth week. (G) Male about 3 months. (H) Male close 

 of fetal life. (I) Female about tenth week. (J) Female about 3 months. (K) Female 

 close of fetal life. (L & M) Stages in development of the broad ligament and separation 

 of the recto-uterine pouch above from the vesico-uterine pouch below. 



