1. The sexual reproduction most common in marine organisms involves releasing both eggs and sperm into water where fertilization can occur. The major problem is that the gametes are diluted by water and union is not ensured without the controls associated with the timing of gamete release. Marine vertebrates ``increase their odds'' by tying spawning to the lunar cycle. The terrestrial environment was hostile to gametes and zygotes, neither of which was resistant to drying out. These problems were combated with shelled eggs.
2. Oviparity is reproduction through laying eggs. In ovoviviparity, the eggs hatch inside the mother, simulating a live birth. Truly viviparous animals are born alive.
3. Monotremes are oviparous. Marsupials have embryos born very underdeveloped and continue development in the pouch; in placentals all development takes place in the mother's body and internal nourishment is provided by means of a network of blood vessels called the placenta.
4. Sex is determined by the presence of the Y chromosome, probably by the SRY gene. The protein produced by this gene turn indifferent gonads into testis. Without the products of this gene, the gonads develop into ovaries.
5. Seminal vesicles contribute a fructose-rich nutrient to the semen, the fluid medium in which sperm travel. The prostate gland is responsible for approximately 60% of the volume of semen. The bulbourethral glands secrete a lubricant to facilitate sexual intercourse.
6. Ova start their meiosis when they are first formed, during embryological development. The first meiotic division is completed during the follicular phase in the Graafian follicle. The second meiotic division is completed after fertilization.
7. The granulosa cells secrete estrogen, which feeds back to the hypothalamus to stop FSH production; estrogen also causes the uterine lining to proliferate. The pituitary responds to estrogen and produces LH that acts at the ovary to inhibit estrogen production and cause the mature follicle to burst and release the egg into the Fallopian tubes.
8. A peak in LH secretion causes ovulation to occur at mid-cycle. The follicle then becomes the corpus luteum and secretes progesterone, which stimulates the further development of the uterine lining. Lack of fertilization after ovulation causes the cessation of progesterone production and ends an individual cycle. Menstruation results from the thickening of the uterus in preparation for the embryo implantation, which must be sloughed off when there is no implantation. Hormonally this layer is lost by a process initiated by a decrease in progesterone levels.
9. The ovum proceeds into the Fallopian tube and travels toward the uterus by peristaltic contractions of the Fallopian tubes. If the oocyte isn't fertilized, it will degrade and pass out with the menstrual flow. If it is fertilized, it will be fertilized in the upper one-third of the Fallopian tube.
10. The four periods are excitement, plateau, orgasm, and resolution. In both the female and male, during excitation there is increased heartbeat, blood pressure, respiration, and vasodilation, as well as increased sensitivity in the nipples and genital tissues. The male experiences vasocongestion in the penis, resulting in erection. The female experiences increased circulation with swelling in the labia and clitoris, and the vaginal walls become moist and its muscles relax.
11. Diaphragms prevent pregnancy by serving as a barrier to sperm, as well as performing a spermicidal function if used in conjunction with spermicidal jellies. Birth control pills prevent the maturation of the egg by providing hormonal signs that ovulation has already taken place. Intrauterine devices probably interfere with the zygote's ability to implant in the uterus, or dislodge it after it implants. None of these three methods prevent STDs.