47.1 Asexual and Sexual Reproduction

• Asexual reproduction occurs without the fusion of gametes, results in offspring with genetic material from only one parent, and en yields offspring that are genetically identical to the parent.

• Sexual reproduction, in contrast，typically involves the fusion of haploid gametes and results in offspring that are genetically different from each other and from their parents.

• There are three main organisms of asexual reproduction.

  • In budding, an offspring begins to form within or on a parent, such as on the hydra.

  • In fission, an individual simply splits into two or more descendants, as shown in anemones.

  • In parthenogenesis (literally，“virgin-origin”)， females develop eggs and produce offspring without any genetic contribution from a male.

    • Parthenogenesis occurs in a diversity of lineages, including certain invertebrates, fishes, lizards, snakes, and birds.

• In budding and fission, females clone themselves一that is, produce large numbers of identical copies of themselves asexually.

• Once the males have matured, sexual reproduction ensues: by meiosis, males produce haploid sperm and females produce haploid eggs, and these gametes fuse to create diploid offspring.

  • Fertilization is the fusion of sperm and egg to form a zygote.

• Oogenesis is the formation of ova (singular: ovum).

  • An ovum is also known as a mature egg.

• In the vast majority of animals, gametogenesis occurs in a sex organ or gonad.

  • Male gonads are called testes; female gonads are called ovaries.

• In the male gonad, diploid cells called spermatogonia (singular: spermatogonium) divide by mitosis.

• The specialized diploid cells produced by spermatogonia are called primary spermatocytes.

  • They undergo meiosis I and produce two haploid secondary spermatocytes, which then undergo meiosis II.

• Each haploid spermatid develops into a spermatozoan (plural: spermatozoa), also commonly known as sperm-a cell that is specialized for carrying a haploid genome from the male through the female reproductive act and fertilizing an egg.

• In the female gonad, diploid cells called oogonia (singular: oogonium) divide by mitosis.

• When the specialized cells produced by an oogonium, the primary oocytes, undergo meiosis, only one of the haploid cells produced can mature into an egg.

  • This secondary oocyte is arrested in meiosis II until it is fertilized by a sperm.

47.2 Reproductive Structures and Their Functions

• In humans, the external anatomy of the male reproductive system, or genitalia, consists of the scrotum and the penis.

• The saclike scrotum holds the testes; the penis functions as the organ of copulation, or sexual intercourse, necessary for internal fertilization.

• A vas deferens is a tube with smooth muscle in its walls that transports sperm from the epididymis to the short ejaculatory duct within the prostate gland, where the sperm are mixed with accessory fluids.

  • The resulting semen then enters the urethra, a longer tube at passes through the penis and services both the reproductive and urinary systems.

  • The semen is expelled from the body during ejaculation.

• Birds are oviparous (“egg-bearing”）animals that lay an amniotic egg protected by a hard shell; most mammals are viviparous (“live bearing”)，and embryonic development takes place entirely within the mother’s body.

• The egg is “ laid” from the cloaca-a chamber that the reproductive, digestive, and excretory systems flow into and that opens to the environment.

• Female genitalia includes the labia minora (singular: labium minus) and the labia majora, the clitoris, the opening of the urethra, and the opening of the vagina.

  • The clitoris is an organ that functions in sexual arousal; it develops from the same population of embryonic cells that gives rise to the penis in males.

47.3 Fertilization and Egg Development

• After males package their sperm into a structure called a spermatophore, which is then picked up and placed into the female’s reproductive tract by the male or the female.

• Follow-up research has confirmed that second-male advantage is widespread, although not universal, in insects and some other animal groups.

• Compounds from the cortical granules cross-link molecules in the vitelline envelope to form a tough fertilization envelope.

  • This impenetrable barrier keeps additional sperm from reaching the sea urchin egg.

• In ovoviviparous species, offspring also develop inside the mother’s body but are nourished by nutrient-rich yolk stored in the egg.

47.4 Embryonic Development

• The stage of rapid cell division that follows fertilization is called cleavage.

• The cells that are created during cleavage are called blastomeres.

• When cleavage is complete, the embryo in many animals consists of a mass of blastomeres called a blastula.

• Cleavage in mammals results in a type of blastula called a blastocyst (“sprout-bag”), which has two populations of cells.

• The exterior of the blastocyst is a thin sheet of cells called the trophoblast (“feeding-sprout”).

  • Inside the trophoblast is a fluid filled cavity (blastocoel) and a cluster of cells called the inner cell mass (ICM).

  • There is a distinction between the trophoblast and the ICM-the embryo develops from the ICM, and the trophoblast forms part of an organ called the placenta.

    • Cells from the mother’s uterus also form part of the placenta.

47.5 The Role of Sex Hormones in Mammalian Reproduction

• In humans this transition called puberty-is directed by increased levels of testosterone in boys and estradiol in girls.

• Day 1 in the menstrual cycle is marked by the beginning of menstruation-the expulsion of the uterine lining.

• The females of these mammals have an estrous cycle and are sexually receptive only during estrus-when they are said to be “in heat."

47.6 Pregnancy and Birth in Mammals

• Pregnancy and lactation-providing milk that nourishes offspring after birth-represent some of the most extreme forms of parental care known in animals.

• As a result, they are far less developed at birth than are the young of eutherian mammals, which undergo a lengthier gestation-the developmental period that takes place inside the mother's uterus.

• Arteries transport blood from the circulatory system of the fetus, through the umbilical cord, to an extensive capillary bed in the placenta.