Determination and Differentiation of Sex
The determination of sex, as pointed out in Chapter VI, probably occurs at the time when the egg is fertilized, depending on the chance combination of the sex chromosomes. The differentiation of sex, on the other hand, or the development of visible distinct sex characters, does not occur in man until about the ninth week of fetal life, although, according to Keith, the testes within the body cavity are distinguishable from the ovaries by the seventh week.
The human external genitalia of the two sexes at first cannot be told apart. At the stage of about the sixth or seventh week, when the legs are simply two stubs on the two sides of the cloaca (Fig. 408), an elevation, the genital eminence, tipped by a conical projection, the genital tubercle, appears just within the anterior margin of the cloacal opening. Around the genital opening there forms a pair of folds, the labio-scrotal folds, which are destined to become either the labia majora of the female or the scrotal sac of the male.
Soon the genital tubercle elongates somewhat and a slit, the urogenital opening, develops along its ventral surface. The lateral margins of the slit elevate next to form a second pair of enclosing folds, the inner genital folds, or the future labia minora of the female.
In the male the genital eminence, with its projecting tubercle, elongates still more into the corpus spongiosum and the glans of the penis, the urogenital slit meanwhile closing over to form the penile part of the urethra. The inner genital folds become the two corpora cavernosa, joining together side by side above the corpus spongiosum to complete the body of the penis, while the labio-scrotal folds draw down and grow together, forming the scrotal sac. The seamlike ridge, or raphe, along the midventral line of the scrotum in the adult, represents the line of fusion between these embryonic structures.
In the female the genital eminence and the genital tubercle remain comparatively undeveloped in the form of the clitoris, while the urogenital slit expands into the vestibular entrance to the vagina, and the two sets of folds on either side shape up into the labia minora and the labia majora. The anterior fusion of the outer labio-scrotal folds, which make the labia majora in the female, becomes the mons Veneris.
So-called human “hermaphrodites” usually present intermediate embryonic features with respect to the external genitalia, as for instance, a small undeveloped penis with an unclosed urogenital slit (hypospadia), resembling the grooved cloacal penis of the turtle.
The general relations of the genitalia, both external and internal, of the two sexes in man are shown in Figures 409 and 410.