Here’s how it happens. About six weeks after the fertilized egg implants in the uterus, there is a wash of masculinizing hormones. The male blastocyst (a group of cells that will form the embryo) responds to this by developing its “prefab” universal genital hardware into the male configuration of penis, testicles, and scrotum. The female blastocyst does not respond to the hormone wash at all, and instead develops its prefab universal genital hardware into the default, female configuration of clitoris, ovaries, and labia.
Welcome to the wonderful world of biological homology.
Homologues are traits that have the same biological origins, though they may have different functions. Each part of the external genitalia has a homologue in the other sex. I’ve mentioned two of them already: Both male and female genitals have a round-ended, highly sensitive, multichambered organ to which blood flows during sexual arousal. On females, it’s the clitoris; on males, it’s the penis. And each has an organ that is soft, stretchy, and grows coarse hair after puberty. On females, it’s the outer lips (labia majora); on males, it’s the scrotum. These parts don’t just look superficially alike; they are developed from the equivalent fetal tissue. If you look closely at a scrotum, you’ll notice a seam running up the center—the scrotal raphe. That’s where his scrotum would have split into labia if he had developed female genitals instead.
Homology is also why both brother and sister will have nipples. Nipples on females are vital to the survival of almost all mammal species, including humans (though a handful of old mammals, such as the platypus, don’t have nipples, and instead just leak milk from their abdomens), so evolution built nipples in right at the very beginning of our fetal development. It takes less energy to just leave them there than to actively suppress them—and evolution is as lazy as it can get away with—so both males and females have nipples. Same biological origins—different functions. the clit, the whole clit, and nothing but the clit The clitoris and penis are the external genital organs most densely packed with nerve endings. The visible part of the clitoris, the glans clitoris, is located right up at the top of the genitals—some distance from the vagina, you’ll notice. (This fact will be crucial when I talk about orgasm, in chapter 8.) The clitoris is . . .
The hokey pokey—it’s what it’s all about.
Two turntables and a microphone—it’s where it’s at.
A Visa card—it’s everywhere you want to be.
It is your Grand Central Station of erotic sensation. Averaging just one- eighth the size of a penis yet loaded with nearly double the nerve endings, it can range in size from a barely visible pea to a fair-sized gherkin, or anywhere in between, and it’s all normal, all beautiful.
Unlike the penis, the clitoris’s only job is sensation. The penis has four jobs: sensation, penetration, ejaculation, and urination.
Two different ways of functioning, one shared biological origin.
The visible part of the clitoris—the glans—is actually just the head of the clit, just as the glans penis—the vaguely acorn-shaped cap at the end of the penile shaft—is just the head. There’s a lot more to it, though. The shaft of the penis is familiar to many. It is constructed of three chambers: a pair of cavernous bodies (corpora cavernosa) and a spongy body (corpus spongiosum), through which the urethra passes. All three of these chambers extend deep into the body. The corpus spongiosum ends in the bulb of the penis deep inside the pelvis. The corpora cavernosa taper away from each other and attach the pelvic bone.
The cultural understanding of clitoris is “the little nub at the top of the vulva.” But the biological understanding of clitoris is more like “far-ranging mostly internal anatomical structure with a head emerging at the top of the vulva.” Like the penis, the clitoris is composed of three chambers: a pair of legs (crura) that extend deep within the tissue of the vulva, which are homologous to the corpora cavernosa, and the bulbs of the vestibule, homologous to the corpus spongiosum, including its bulb of the penis. The vestibule is the mouth of the vagina; the bulbs extend from the head of the clitoris, deep inside the tissue of the vulva, then split to straddle the urethra and the vagina. That’s right: The clitoris extends all the way to the vaginal opening.
The anatomy of the clitoris. The cultural meaning of “clitoris” is often limited to the external part, the glans. The biological meaning includes a vast range of internal erectile tissue that extends all the way to the vaginal opening.
The anatomy of the penis. As with the clitoris, the cultural meaning of “penis” is limited to the external part—the glans and shaft. And, like the clitoris, the penis has internal erectile tissue. All the same parts, organized in different ways.