320 CHORDATE ANATOMY 



Subsequent changes involve the attachment of the tubule to the adjacent 

 collecting tubule, the elongation of the excretory tubule, and the ingrowth 

 of a glomerulus into the Bowman's capsule. Arterial and venous con- 

 nexions are subsequently estabhshed similar to those of the mesonephros. 

 (Fig. 282) These changes occur in the later months of intra-uterine hfe. 

 Connexions with the Bladder. During the earher stages of its develop- 

 ment, each ureter shares with a mesonephric duct a common lateral 

 opening into the cloaca. Between 10 mm. and 17 mm. stages, the cloaca 

 becomes divided by a septum into a dorsal rectum and a ventral urogenital 

 sinus. The septum, which is completed during the seventh week, becomes 

 the perineum of the adult. After the septum is formed the mesonephric 

 ducts and ureters retain their connexion with the urogenital sinus. Even 

 before the development of the septum is completed, in an 11 mm. embryo, 

 the urogenital sinus becomes subdivided into a vesico-urethral portion 

 into which the ureter and mesonephric ducts enter, and a phalUc portion 

 which extends into the genital tubercle. By the time the embryo attains 

 a length of 25 mm. (2 months), the ureters and mesonephric ducts are 

 separated, the ureters opening into the bladder and the mesonephric ducts 

 into the urethra. The bladder, therefore, arises not from the allantois 

 but from the cloaca. The bladder anlage, however, is continued ventrally 

 as the allantoic stalk, which subsequently atrophies to form the middle 

 umbiUcal ligament. 



B. Reproductive Organs 



The human embryo is usually characterized as sexually indifferent. 

 This popular view is based upon the similarity of the anlagen of male and 

 female reproductive glands in the early embryo, and upon the occasional 

 appearance of hermaphroditic adult individuals. According to modern 

 genetical opinion, however, sex is definitely predetermined in the fertilized 

 egg and is only exceptionally modifiable. Nevertheless, ovaries and 

 testes develop from morphologically similar genital folds, located between 

 the mesonephroi and the mesentery. At their first appearance, the genital 

 folds are elongated masses of epitheUal cells which become differentiated 

 into an external many-layered epithelium and an inner epithelial mass 

 derived from the peritoneum. Together with the mesonephros, the 

 gonad forms a urogenital ridge, the prominence of which is increased by 

 the growth of underlying adrenal tissue. (Figs. 66, 67, 281) 



As the genital folds increase in size, longitudinal grooves develop 

 separating them from the lateral mesonephros and the median mesentery. 

 Connexion of the gonad with the mesonephros and with the body-wall 

 is finally reduced to a thin mesentery-like membrane. In the male, this 

 membrane forms the mesorchium and in the female the mesovarium 

 through which blood and nervous connexions are retained. 



