Reproduction

∞ generated and posted on 2016.04.19 ∞

Means by which organisms generate offspring.

Human reproduction involves a combination of sperm and ovum production, sperm delivery into a vagina, sperm travel into the fallopian tubes for ovum fertilization, fertilization and subsequent development, implantation, further development of the embryo then fetus, parturition, breastfeeding, and all the rest.

Mammals share a common pattern for the embryology of sexual organs… The early embryo is sexually indifferent and contains all precursors and structures necessary for development of either male or female organs. After about the eighth week following conception, the gonads begin to differentiate as either ovaries or testes. The developing testes secrete androgens, which induce the development of male genitalia. If androgens are absent, or present at low levels, female genitalia are formed.

The internal and external genitalia develop in different ways. For internal genitalia, the early embryo contains precursors of both sexes: the Müllerian ducts (which form the Fallopian tubes and ovaries of females) and the Wolffian ducts (which form the vas deferens—the ducts that carry down from the testes to the penis—in males). In females, the Wolffian ducts degenerate and the Müllerian ducts differentiate; males develop by the opposite route.

The external genitalia follow a markedly different pattern. Individuals do not begin with two distinct sets of precursors and then lose one while strengthening the other. Rather, the different organs of male and female develop along diverging routes from the same precursor. The male's penis is the same organ as the female's clitoris—they form from the same tissues, are indistinguishable in the early embryo, and follow different pathways later. The male's scrotum is the same organ as the female's labia majora. The two lips simply grow longer, fold over and fuse along the midline, forming the scrotal sac.

The female course of development is, in a sense, biologically intrinsic to all mammals. It is the pattern that unfolds in the absence of any hormonal influence. The male route is a modification induced by secretion of androgens from the developing testes. — Stephen Jay Gould, 1991, from Bully for Brontosaurus, pp. 153-154

The above video considers sex with very much a human though not only a human perspective.

The above video looks at reproduction more generally over much of the video, so this provides a good introduction to variation in reproductive strategies as seen between different types of organisms.


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