Sexual Reproduction Notes


Stranger than truth!

Angler Fish-

Angler Fish Reproduction


Fertilization: the fusion of DNA from two sex cells called a _gamete_ (a sperm or egg cell) creates the first single cell of new life.

  • The first cell of life is called a _zygote_.
  • If the sperm and egg are produced by the same individual it is called a _hermaphrodite_.

ex. earthworms

  • Sometimes the sex of an organisms can change AFTER they are born.

ex. _clownfish_


Internal Fertilization vs. External Fertilization

External Fertilization: Sperm is deposited near eggs in water and _swim_ to fertilize the egg. Embryo develops _externally_.

Common in _fish_ and amphibians


    1. _Many_ eggs and sperm are released
    2. water is required (also keeps egg from drying out)
    3. nutrients come from _egg_
    4. little parental care
    5. _low_ survival rate

Internal Fertilization: Meeting of the sperm and egg occurs _inside_ the female species. Embryo can develop internally OR externally (shelled egg is _laid externally_).

Common in reptiles, mammals and _birds_

    Oviparous organisms, including most insects, reptiles and all birds lay eggs that continue to develop after being laid, and _hatch_ later.

    Viviparous organisms, including almost all mammals (such as whales, kangaroos and humans) bear their young _live_.

  • Characteristics:
    1. _few_ eggs
    2. terrestial environment
    3. nutrients come from _mother or egg_
    4. much parental care
    5. _high_ survival rate


Feynman on Beauty- stay curious

Bird's Courtship behavior



somatic cell
sex cell
egg cell
sperm cell
internal fertilization
external fertilization

Essential Questions:

1. In what ways do creatures that reproduce by internal fertilization different from those who practice external fertilization? Be sure to include number of eggs, location of fertilization, where the offspring is located and degree of parental care.


2. Which animals undergo internal fertilization and which fertilize externally? Give three examples of each.


3. Why is fertilization between two different individuals superior to having offspring from a single parent.



Female Reproductive System

Female Reproductive Structures

Gametes (in this case _eggs_) are produced by ovaries.

Oviducts (_fallopian_ tubes) serve as passageways from ovaries to uterus. Fertilization occurs here but fetal development occurs in the uterus.

The uterus is a muscular sack that sits on top of the bladder. It is here that _embryo_ develop. The lining is shed during menstruation.

The cervix is muscular _opening_ between uterus and vagina.

The vagina is a muscular canal that functions as a _birth_ canal. It opens between urethra and anus.

The _exterior_ genitalia of females are called the vulva.

The clitoris is _analogous_ to the penis in males; it contains erectile tissue. The external genetalia of the female and male look identical up to about 8 weeks.

Labia are folds of skin near the opening of the vagina. Labia majora are fleshy and covered with pubic hair. They are homologous with the _testes_ in the male.

Hormones and the menstrual cycle

Estrogen is a hormone created in the _ovaries_. It causes the development of _secondary_ sex characteristics. ex.

The menstrual cycle is a hormone-controlled monthly cycle in the human female. It lasts _28_ days. During the menstrual cycle, an egg is released from an _ovary_ at midcycle and the uterine lining thickens in preparation for possible pregnancy.

It is divided into four stages:

  1. follicle stage: the egg is _maturing_
  2. _ovulation_: when the egg is released from the ovary
  3. corpus luteum stage: the remains of the _follicle_ after the egg left.
  4. _menstruation_: the monthly shedding of the lining

Only humans and other primates have a menstrual cycle. Other mammals have an estrous cycle which is marked by periodic changes in the females's sex organs and in the desire to mate.

  • Reproductive cycles in many vertebrates and invertebrates are related to changes in day length. Female humans are NOT in this category!

If the egg is unfertilized, _menstruation_ occurs and the uterine lining is shed.

If the released egg is _fertilized_, no menstration occurs.


  • Around age 45-50, the ovaries gradually stop responding to pituitary hormones (FSH, LH).
  • As a result, follicle production stops and the menstrual cycle _stops_.



oviducts (fallopian tubes)
menstrual cycle
follicle stage
corpus luteum
secondary sex characteristics

Essential Questions:

1. What are the functions of the ovaries?


2. What is the function of estrogen?


3. Trace the path of the unfertilized egg as it leaves the ovary.


4. What are the stages of the menstrual cycle?


Male Reproductive System

Male Reproductive Structures Diagram

Video on the Male Reproductive System (Note: the video cuts off at the end but it is still very informative!)

Sperm is produced in the _testes_.

    • The testes are located in the _scrotum_ ; a sac outside the body. Sperm require a _lower_ body temperature than the body core.

Sperm travels from the testes through the _vas_ deferens tube into the urethra and finally out through the penis

  • The urethra is a tube that carries _urine_ from the bladder to outside the body.
  • In males, the urethra also carries _sperm_.

Semen- fluid added to the sperm to nourish and _hydrate_ the sperm for the journey! Only 1% of the semen is sperm!

  • The prostate gland produces a secretion that is basic in pH which neutralizes the acidity of the vagina (which is normally a pH of _3.8_ to 4.5).
  • The seminal vesicles add _glucose_ to feed the sperm and make up 60% of the volume of semen.
  • The cowpers gland (bulbourethral gland) produces a high pH fluid that neutralizes the acidity of the vagina and the _urinary_ residue in the urethra.

The Sperm Release Pathway

An average male produces approximately 1,000 sperm/second (30 billion/year).

Each ejaculation should contain 200-300 million sperm. Sperm counts below 10 to 20 million per milliliter usually present fertility problems.


Testosterone is the hormone responsible for the development and function of the male reproductive tract. It is created in the _testes_.

  • The formation of _sperm_ is stimulated by testosterone.
  • Testosterone promotes growth.
  • It also stimulates the development of _secondary_ sexual characteristics such as beard growth, _lowering_ of the voice, etc.



vas deferens
prostate gland
seminal vesicle
Cowper's gland
secondary sex characteristics

Essential Questions:

1. What are the functions of the testes?

2. What is the function of testosterone?

3. Trace the path of sperm as it leaves the testes.


Life's Greatest Miracle