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Comparative Morphology of Chordates 



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Fig. 198. Development of the male external genitalia of man. (A) Indifferent 

 stage from which either sex may develop. (B) Early stage. (C) Later stage, (a) 

 Anus; (/) genital folds; (</) urethral groove; (p) genital tubercle; (r) genital ridge 

 (outer genital folds); (s) scrotum. (After Hertwig. Courtesy, Kingsley: "Com- 

 parative Anatomy of Vertebrates," Philadelphia, The Blakiston Company.) 



extends to the distal tip of the genital tubercle. Meanwhile, growth of 

 the internal genital folds causes them to arch over the cloacal slit and 

 urethra] groove, enclosing them to become the urethral canal, which 

 finalh retains communication with the exterior only by a small orifice, 

 the meatus urinarius, on the distal surface of the glans. The mesen- 

 chyme within the internal genital folds of the phallus differentiates 

 into a corpus cavernosum penis (more or less definitely divided 

 into right and left corpora) ventral to the urethra, and another spongy 

 body, the corpus cavernosum urethrae or corpus spongiosum, a 

 median body dorsal to the corpus cavernosum penis and completely 

 surrounding the urethra (Figs. 492, 499). Distally, the dorsal corpus is 

 continuous with the glans and its proximal end forms a bulblike en- 

 largement, the bulbus urethrae. At the base of the adult penis the 

 two halves of the corpus cavernosum penis diverge, forming the 

 crura, each arm (crus) of which extends forward and usually becomes 

 attached to the posterior border of the adjacent ischium, thus giving 

 the organ a skeletal anchorage. In many mammals a longitudinal rod 

 of bone is formed in the connective tissue between the paired corpora 

 cavernosa. This os penis is commonly found among rodents, bats, 

 carnivores, and whales, and in some monkeys and apes. The skin at 

 the distal end of the penis forms a cylindric fold, the foreskin or 

 prepuce, surrounding the glans. 



The external genital folds (Fig. 498B, C) initiate the development 

 of the scrotum, but the greater part of it results from an outward 

 protrusion of a median area of perineum just dorsal (i.e., toward the 

 anus) to the developing penis. 



The embryonic parts which produce the male external genitalia are 



