Capulation and Fertilization

For fertilization to occur,sperm must be deposited in the vagina within a few (5) days before or on the day of ovulation.Sperm transfer is accomlished by copulation.Sexual excitation dilates the arterioles supplying blood to the penis.Blood accumulates in three cylindrical spongy sinuses that  run lengthwise through the penis.The resulting pressure causes the penis to enlarge and erect and thus able to penetrate the vagina.

Movement of the penis back and forth within the vagina causes sexual tension to increase to the point of ejaculation.Contraction of  the walls of  walls of  each vas deferens propels the sperm along. .Fluid is added to the sperm by the seminal vasicles,Cowper’s glands,and the prostate gland.These fluids provide.

·         a source of energy  (fructose)

·         an alkaline enviroment to activate the sperm,perhaps in other ways provide an optimum g environment for them.

The mixture of sperm and accessory fluids is called semen.it passes through the urethra and is expelled into the vagina.Physiological changes occur in the female as well as the male in response to sexsual ecitement,although these are not as readily apparent.

Although sperm can swim several millimeters each second,their trip to and through the follopian tubes may be assisted by muscular contraction of the walls of the uterus and the tubes.There is also evidence that they respond to a chemical attractant produced by the egg or the egg or the tissues surrounding it.

Within moments,enzymes released from the egg cytosol act on the zona making it impermeable to the other sperm that arrive.

Soon the nucleus of the successful sperm enlarges into the male pronucleus.At the same time,the egg (secondary oocyte) completes meiosis II forming a second polar body and the female pronucleus.The male and female pronuclei move toward each other while duplicating their DNA in  Sphase ...Their nuclear enveiopes disintegrate),and a full set of dyads assembles on it.In sea urchins,at least, the block to additional sperm entry and the fusion of the pronuclei are triggered by nitric oxide generated in the egg by the sperm acrosome.