662 



Comparative Morphology of Chordates 



Embryo 



Genital tubercle 



Internal genital folds 



Urinogenital space and 



urethral groove 

 External genital folds 



Adult Male 



Glans penis and Corpus 



spongiosum 

 Corpora cavernosa penis 



Penile urethra 



Scrotum (in part) 



Adult Female 



Glans clitoridis 



Corpora cavernosa clitor- 

 idis and Labia minora 

 Vestibule 



Labia majora 



The development of the human external genitalia begins early in 

 the second month of fetal life and is well advanced late in the fourth 

 month. Or, in terms of fetal size, their development is seen in a series 

 of fetuses ranging from 10 to 150 mm. in all-over length. 



The copulatory structures of monotremes are more reptilian 

 than mammalian. When not erected, the penis is entirely withdrawn 

 into the cloaca (Fig. 496). The urethral (seminal) groove, extending 

 between corpora cavernosa, is closed over dorsally except at its anterior 

 end, where it remains open into the cloaca, but no definite corpus 

 cavernosum urethrae is formed. During erection, however, the anterior 

 aperture is closed so that, temporarily, there is a continuous and exclu- 

 sive seminal passage from the vasa deferentia to the exterior. The 

 monotreme "urethra," therefore, is not physiologically equivalent to 

 the urethra of a placental mammal. A median furrow divides the glans 

 into right and left parts, each beset with numerous small spines. The 

 urethra divides distally into two branches, one passing to each half 

 of the glans, where it subdivides into several fine ducts, each opening 

 at the apex of a spine. 



The marsupial penis is essentially like that of a placental mam- 

 mal but, when not erected, is withdrawn into the shallow ectodermal 

 cloaca with its distal end directed backward (Fig. 496). The crura of 

 the corpora cavernosa penis are not attached to the pelvic skeleton. 

 In many cases — e.g., the opossum (Didelphys) and bandicoot (Pera- 

 meles) — the glans is two-forked in adaptation to the female's paired 

 vaginas. The urethra may branch correspondingly, or from its median 

 aperture a superficial groove may extend to the tip of each fork of the 

 glans. In a species in which the male glans is two-forked, the clitoris like- 

 icise has a bifid ylans. The scrotum, not present in all marsupials, is 

 anterior to I he penis, a position probably made necessary by the fact 

 thai the perineal space is of small extent and depressed below the 

 surrounding surface of the body. 



In placental mammals, the more extensive perineal area makes 

 possible the more advantageously placed postpenial scrotum, and the 



