202 



UROGENITAL SYSTEM 



part of the now narrow cloacal chamber, and lies at the bottom of a shallow 

 transverse depression (ano-perineal area). 



Owing to the increase of the mesoderm round the base of the cloacal elevation, 

 the lips of the outer depression become raised into the outer genital folds. The 

 increase is, however, greatest at the anterior or cranial lip, and the salient angle 

 already alluded to grows in a ventral direction to form the genital papilla. The 

 cloacal plate is necessarily displaced ventrally and rotated until it lies in the position 

 seen in fig. 253, D, on the caudal face of the genital papilla, as a mesial ridge which 

 runs, at the apex of the papilla, into an epithelial horn (seen in fig. 254, B) 

 produced by a thickening of the surface-ectoderm. 





g.p. 

 i.g.f. 





cut. 



lab min. - 



lab maj. 



hum." 





:. H 



. 254. DEVELOPMENT OP THE EXTEENAL OBGANS. A, MALE EMBRYO OF 23 MM. LONG; B, MALE 



EMBBYO OP 29 MM. LONG; C, FEMALE FO3TUS OF 7 MM. LONG (ELEVENTH WEEK); D, FEMALE 

 F03TUS OF 15 CM. LONG (SIXTEENTH WEEK). 



gp, genital papilla ; gls, glans ; clit, clitoris ; o.g.f., outer genital folds ; i.g.f., inner genital folds ; 

 an, anus; sin, urogenital sinus; lab, min. } labia minora; lab. maj., labia majora ; liyrti, hymen; 

 cc, coccyx; um.c., umbilical cord. 



The cloacal plate at the base of the genital papilla now breaks through, and 

 the urogenital opening is established. By a loosening of the central cells of the 

 plate in front of the opening a groove is produced, running on to the caudal face 

 of the genital papilla, and known as the urethral groove. The edges of this groove, 

 formed by the salient lateral sheets of mesoderm, form the inner genital folds. The 

 cloacal chamber has meantime been completely divided, and the mesoderm, which 

 lies between the dorsal and ventral portions of it, is allowed to reach the surface, 

 and here forms the perineum. A ring-shaped mesodermic thickening at the same 

 time fosms round the proctodaeal depression. This at first has the form of two 

 projections from the anal tubercle, which grow forwards round the anal depression, 

 and gradually convert it from a transverse into a semilunar, then into a circular, 



