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The physical difference between the sexes is not as great as it may appear at first glance. Even the female and male sex organs are structurally very similar. Indeed, in the first weeks of their development they are practically indistinguishable. It is only their subsequent growth before birth and later during puberty which makes them complementary to each other for the purpose of procreation. However, even then one can still find corresponding, i.e. "homologous" structures that originated in the same cell mass.

出生前

兩性都來自於公共的胚基

在生命的早幾周裏，男女胚胎仍保持未分化狀態。這時，他們都有了頭和肢芽。肢芽是上肢和下肢的前驅結構。同樣，這些胚胎也有將要生長成性器官的組織脊，既生殖脊。可是，在這個生殖脊上，人們所能夠見到的全部就是一個隆起（使人聯想到男性）和一條凹溝（讓人聯想到女性）。

典型的女性發育

對於女性胚胎，這個隆起發育成陰蒂。而凹溝仍敞開著，逐漸發育形成小陰唇和引導的前部分。在兩側，兩小片皮膚發育成大陰唇。另有一些細胞團發育成卵巢。在出生時，外部和內部性器官讓人能夠清晰地分辨出女性。

在青春期

女性性器官只有在青春期，一般在11-13歲之間，才開始發育直到成人大小和具有生殖功能。從這個意義上看，性成熟的標誌是陰毛和首次月經來潮。

男孩

出生前

出生前

兩性都來自於公共的胚基

在生命的早幾周裏，男女胚胎仍保持未分化狀態。這時，他們都有了頭和肢芽。肢芽是上肢和下肢的前驅結構。同樣，這些胚胎也有將要生長成性器官的組織脊，既生殖脊。可是，在這個生殖脊上，人們所能夠見到的全部就是一個隆起（使人聯想到男性）和一條凹溝（讓人聯想到女性）。

典型的男性發育

對於男性胚胎，這個隆起發育成陰莖，而凹溝則封閉起來。另外的細胞團發育成睾丸，隨後開始分泌睾丸激素。睾丸激素繼爾刺激生殖脊進一步發育和分化（缺了睾丸激素，胚胎必將“自動地”發育成一個女胎）。在生殖隆起兩側的兩小片皮膚形成陰囊。到了妊娠的第7-9月，睾丸降入陰囊內（假如未降入陰囊，則需要後來的矯正，以避免不育）。在出生時，外部和內部性器官讓人能夠清晰地分辨出男性。

在青春期

男性性器官只有在青春期，一般在12-17歲之間，才開始發育直到成人大小和具有生殖功能。從這個意義上看，性成熟的標誌是陰毛和首次射精。

Girls

Before birth

Both sexes start from a common base
Female and male embryos remain undifferentiated during the first weeks of their lives. They already have a head and limb buds which begin to grow into arms and legs. They also have a ridge of tissue destined to grow into sex organs. However, all one can see at this point is a hump (suggestive of the male) and a groove (suggestive of the female).

The female sex organs reach their full size and function only during puberty, usually between the ages of 11-13. Indications of sexual maturity in this sense are the growth of pubic hair and the first menstruation.

Boys

Before birth

Both sexes start from a common base
Female and male embryos remain undifferentiated during the first weeks of their lives. They already have a head and limb buds which begin to grow into arms and legs. They also have a ridge of tissue destined to grow into sex organs. However, all one can see at this point is a hump (suggestive of the male) and a groove (suggestive of the female).

Typical male development
In the male embryo the hump grows into the penis, and the groove closes. Another cell mass grows into testicles which begin to produce the hormone testosterone. This hormone, in turn, stimulates further growth and differentiation. (Without it, the embryo would “automatically” grow into a female.) Two patches of skin on either side of the hump form the scrotum. Between the seventh and ninth month of pregnancy, the testicels descend into the scrotum. (If they fail to descend, some later correction is required in order to prevent infertility.) At birth, the internal and external sex organs are clearly defined as male.

During puberty

The male sex organs reach their full size and function only during puberty, usually between the ages of 12-17. Indications of sexual maturity in this sense are the growth of pubic hair and the first ejaculation of semen.