124 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



hypospadias is complete then probably the internal sexual organs 

 have been arrested in their development, and the sex of the 

 individual can be determined only by the microscopic examina- 

 tion of the genital glands. The development of the prostatic 

 and membranous parts of the urethra from the uro-genital sinus 

 has been already dealt with (see Figs. 91, 93, and 99). 



The Scrotum is formed during the fourth month by the union 

 of the external genital folds (labia majora in the female) between 

 the penis in front and the perineal body behind. The line of 

 union is marked by the raphe. 



The Fate of the Allan tois (Fig. 99). — The part of the, 

 allantois outside the abdominal parietes forms (1) the inner 

 or vascular layer of the chorion (p. 97), and part of the umbilical 

 cord (Figs. 75 and 78). The cavity of the allantois within the 

 cord disappears by the end of the third month. 



The intra-abdominal part of the allantois forms : 



(1) The Urachus between the umbilicus and bladder (Fig. 99). 

 It is reduced to a fibrous cord before birth. It may remain 

 patent in the middle and closed at both ends and thus give rise 

 to a urachial cyst behind the abdominal wall. Or it may open at 

 the umbilicus, or at the bladder or at both, thus giving rise to a 

 urinary fistula at the umbilicus. Urachial cysts are rare. 



(2) The apical part of the bladder. 



PARTS FORMED FROM THE CLOACA. 



(1) The Bladder. It was formerly customary to describe the 

 bladder as a derivative of the basal part of the allantois but more 

 recent researches show it to be part of the cloaca (Figs. 95 

 and 108). 



At birth the neck of the bladder lies above and behind the 

 symphysis pubis (Fig. 99). The cavity of the bladder is then 

 fusiform in shape and mostly extra-pelvic in position. It differs 

 from the adult bladder in having a very small trigone and its cavity, 

 therefore, a very short posterior limb in the closed condition. 



(2) The Female Urethra and the corresponding part in the 

 male, viz., the part between the neck of the bladder and the sinus 

 pocularis. 



