THE REPRODUCTIVE SYSTEM AND EMBRYONIC DEVELOPMENT - Animal Structure and Function - Cracking the AP Biology Exam

Cracking the AP Biology Exam


Animal Structure and Function


Reproduction in animals involves the production of eggs and sperm.

Before we launch into reproduction, let’s take a look at something that ties in very nicely with everything we’ve just discussed about hormones and the endocrine system: the menstrual cycle.

Since the ovaries release hormones, they are considered endocrine glands. The ovaries have two main responsibilities:

  • They manufacture ova.
  • They secrete estrogen and progesterone, sex hormones that are found in females.

The hormones secreted by the ovaries are involved in the menstrual cycle.


Phase 1: The Follicular Phase

In phase 1, the anterior pituitary secretes two hormones: follicle-stimulating hormone (FSH) and luteinizing hormone (LH). The FSH stimulates several follicles in the ovaries to grow. Eventually, one of these follicles gains the lead and dominates the others, which soon stop growing. The one growing follicle now takes command.

Because the follicle is growing in this phase, the phase itself is known as the follicular phase. Remember that during all this time the follicle is releasing estrogen. Estrogen helps the uterine lining to thicken and eventually causes the pituitary to release LH. This increase in estrogen causes a sudden surge in luteinizing hormone. This release of LH is known as a luteal surge. LH triggers ovulation—the release of the follicle from the ovary.

There are thus three hormones associated with the follicular phase:

  • Follicle-stimulating hormone (FSH)—originates in the pituitary gland.
  • Estrogen—originates in the follicle.
  • Luteinizing hormone (LH)—originates in the pituitary gland.

The luteal surge makes the follicle burst and release the ovum. The ovum then begins its journey into the fallopian tube, which is also known as the oviduct. This is a crucial event in the female menstrual cycle and is known as ovulation. Once the ovum has been released, the follicular phase ends and the ovum is ready to move on to the next phase.

In addition to the growth of the follicle, the follicular phase involves the thickening of the uterine walls, or endometrium. This happens in preparation for the implantation of a fertilized cell. The entire follicular phase lasts about 10 days.

Phase 2: The Luteal Phase

By the end of the follicular phase, the ovum has moved into the fallopian tube and the follicle has been ruptured and left behind in the ovary. However, the ruptured follicle (now a fluid-filled sac) continues to function in the menstrual cycle. At this stage, it condenses into a little yellow blob called the corpus luteum, which is Latin for “yellow body.”

The corpus luteum continues to secrete estrogen. In addition, it now starts producing the other major hormone involved in female reproduction, progesterone. Progesterone is responsible for readying the body for pregnancy. It does this by promoting the growth of glands and blood vessels in the endometrium. Without progesterone, a fertilized ovum cannot latch onto the uterus and develop into an embryo. We can therefore think of progesterone as the hormone of pregnancy.

After about 13 to 15 days, if fertilization and implantation have not occurred, the corpus luteum shuts down. Once it has stopped producing estrogen and progesterone, the final phase of the menstrual cycle begins.

Phase 3: The Flow Phase, or Menstruation

Once the corpus luteum turns off, the uterus can no longer maintain its thickened walls. It starts to reabsorb most of the tissue that the progesterone encouraged it to grow. However, since there is too much to reabsorb, a certain amount is shed. This “sloughing off,” or bleeding, is known asmenstruation.

With the end of menstruation, the cycle starts all over again, readying the body for fertilization. Let’s recap some of the major steps:

1. In the follicular phase, the pituitary releases FSH, causing the follicle to grow.

2. The follicle releases estrogen, which helps the endometrium to grow.

3. Estrogen causes the pituitary to release LH, resulting in a luteal surge.

4. This excess LH causes the follicle to burst, releasing the ovum during ovulation.

5. The shed follicle becomes the corpus luteum, which produces progesterone.

6. Progesterone, the “pregnancy hormone,” enhances the endometrium, causing it to thicken with glands and blood vessels.

7. If fertilization does not occur after about two weeks, the corpus luteum dies, leading to menstruation—the sloughing off of uterine tissue.

If pregnancy occurs, the extraembryonic tissue of the fetus releases human chorionic gonadotropin (HCG), which helps maintain the uterine lining. The AP Biology Exam is likely to contain questions dealing with the events that occur during the menstrual cycle and fertilization and where those events take place.

Take a look at the following diagram. Familiarize yourself with the parts of the female reproductive system and pay special attention to the different sites of the stages we’ve just discussed.


  • The follicles (and thus the ova) are contained in the ovaries.
  • Hormones are released from the ovaries and pituitary gland.
  • Fertilization occurs in the fallopian tube.
  • The fertilized ovum implants itself in the uterus.


Now let’s discuss the hormones in the male reproductive system. Testosterone, along with the cortical sex hormones we saw earlier, is responsible for the development of the sex organs and secondary sex characteristics. In addition to the deepening of the voice, these characteristics include body hair, muscle growth, and facial hair, all of which indicate the onset of puberty. Testosterone also has another function. It stimulates the testes, the male reproductive organs, to manufacture sperm cells. Testosterone does this by causing cells in the testes to start undergoing meiosis.

Take a look at the male reproductive system.

Sperm and male hormones are produced in the testes. The main tissues of the testes, called the seminiferous tubules, are where spermatogonia undergo meiosis. The spermatids then mature in the epididymis. The interstitial cells, which are supporting tissue, produce testosterone and other androgens. Sperms then travel through the vas deferens and pick up fluids from the seminal vesicles (which provides them with fructose for energy) and the prostate gland (which provides an alkaline fluid that neutralizes the vagina’s acidic fluids). Semen is transported to the vagina by the penis.

Unlike the female reproductive system, the male reproductive system continues to secrete hormones throughout the life of the male. FSH targets the seminiferous tubules of the testes, where it stimulates sperm production. LH stimulates interstitial cells to produce testosterone.



follicle-stimulating hormone (FSH)

luteinizing hormone (LH)

follicular phase

luteal surge


fallopian tube


uterine walls


corpus luteum


human chorionic gonadotropin (HCG)


sperm cells

seminiferous tubules


interstitial cells

vas deferens

seminal vesicles

prostate gland


How does a tiny, single-celled egg develop into a complex, multicellular organism? By dividing, of course. The cell will change shape and organization many times by going through a succession of stages. This process is called morphogenesis. In order for the human sperm to fertilize an egg it must dissolve the corona radiata, a dense covering of follicle cells that surrounds the egg. Then the sperm must penetrate the zona pellucida, the zone below the corona radiata.

When an egg is fertilized by a sperm, it forms a diploid cell called a zygote:

Fertilization triggers the zygote to go through a series of rapid cell divisions called cleavage. What’s interesting at this stage is that the embryo doesn’t grow. The cells just keep dividing to form a solid ball called a morula:

One cell becomes two cells, two cells become four cells, etc.


The next stage is called blastula. As the cells continue to divide, they “press” against each other and produce a fluid-filled cavity called a blastocoel:


During gastrulation, the zygote begins to change its shape. Cells now migrate into the blastocoel and differentiate to form three germ layers: the ectoderm, mesoderm, and endoderm:

  • The outer layer becomes the ectoderm.
  • The middle layer becomes the mesoderm.
  • The inner layer becomes the endoderm.

Each germ layer gives rise to various organs and systems in the body. Here’s a list of the organs that develop from each germ layer.

  • The ectoderm produces the epidermis (the skin), the eyes, and the nervous system.
  • The endoderm produces the inner linings of the digestive tract and respiratory tract, as well as accessory organs such as the pancreas, gall bladder, and liver. These are called “accessory” organs because they are offshoots of the digestive tract, as opposCed to the channels of the tract itself.
  • The mesoderm gives rise to everything else. This includes bones and muscles as well as the excretory, circulatory, and reproductive systems.


The neurula stage begins with the formation of two structures; the notochord, a rod-shaped structure running beneath the nerve cord, and the neural tube cells, which develop into the central nervous system. By the end of this stage, we’re well on our way to developing a nervous system:

As far as the AP Biology Exam is concerned, the order of the stages and the various events is extremely important. For our purposes, think of the order of embryological development in this way:

Zygote → Cleavage → Blastula → Gastrula → Organogenesis

What About Chicken Embryos?

In addition to the primary germ layers, some animals have extraembryonic membranes. Your average developing chicken, for example, possesses these membranes.

There are basically four extraembryonic membranes: the yolk sac, amnion, chorion, and allantois. These extra membranes are common in birds and reptiles.

For the AP Biology Exam, you should be familiar with these membranes and their functions.

Fetal Embryo

The fetal embryo also has extraembryonic membranes during development: the amnion, chorion, allantois, and yolk sac. The placenta and the umbilical cord are outgrowths of these membranes. The placenta is the organ that provides the fetus with nutrients and oxygen and gets rid of the fetus’s wastes. The placenta develops from both the chorion and the uterine tissue of the mother. The umbilical cord is the organ that connects the embryo to the placenta.

Development of an Embryo

During embryonic development, some tissues determine the fate of other tissues in a process called induction. Certain cells, called organizers, release a chemical substance (a morphogen) that moves from one tissue to the target tissue. It is now known that development involves many episodes of embryonic induction.

Homeotic genes control the development of the embryo. Some homeotic genes, called homeobox genes, consist of homeoboxes (short, nearly identical DNA sequences) that encode proteins that bind to DNA; these proteins tell cells in various segments of the developing embryo what type of structures to make. The process by which a less specialized cell becomes a more specialized cell type is called differentiation. Interestingly, homeobox genes are shared by almost all eukaryotic species. Hox genes, which are a subset of homeobox genes, specify the position of body parts in the developing embryo. Mutations in Hox genes result in the conversion of one body part into another. For example, in Drosophila, a specific Hox mutation results in a leg developing where an antenna would normally be.

The cytoplasm can also have an influence on embryonic development. For example, chicken and frog embryos contain more yolk in one pole (the vegetal pole) versus the other pole (the animal pole). This causes cells within the animal pole to divide more, and thus to be smaller than those of the vegetal pole. Because of the distribution of the yolk, cleavage of the egg does not produce eggs that develop normally. If the egg is divided into an animal and vegetal pole, development does not proceed normally.


Directions: Each of the questions or incomplete statements below is followed by five suggested answers or completions. Select the one that is best in each case. Answers can be found here.

1. Which of the following processes produces embryonic germ layers?

(A) Gastrulation

(B) Cleavage

(C) Blastulation

(D) Organogenesis

(E) Fertilization

2. In mammals, all of the following are associated with embryonic development EXCEPT

(A) infolding and clustering of cells leading to the formation of the notochord

(B) the influence of embryonic cells in the differentiation of neighboring cells

(C) a series of rapid meiotic divisions during each cell cycle

(D) the movement of embryonic cells from the surface of the embryo to an interior location

(E) the development of extraembyonic membranes

3. Which of the following is derived from embryonic endoderm?

(A) Epidermis of skin

(B) Muscular system

(C) Skeletal system

(D) Stomach

(E) Pancreas

4. The reduction in cell size from zygote to blastula in mammals is most likely due to:

(A) the loss of DNA in the embryo

(B) decreases in the amount of cytoplasm per cell

(C) feedback inhibition

(D) the haploid nuclei of the embryo

(E) the formation of the blastocyst

Directions: Each group of questions consists of five lettered headings followed by a list of numbered phrases or sentences. For each numbered phrase or sentence, select the one heading that is the most closely related to it and fill in the corresponding oval on the answer sheet. Each heading may be used once, more than once, or not at all in each group.

Questions 5–7 refer to the following male reproductive organs

(A) Interstitial cells

(B) Vas deferens

(C) Seminal vesicles

(D) Prostate gland

(E) Seminiferous tubules

5. Highly coiled tube in which sperms are produced

6. Secrete a fructose-rich fluid that serves as an energy source for sperm

7. Secretes male sex hormones

8. Which of the following extraembryonic membranes stores waste products?

(A) chorion

(B) amnion

(C) allantois

(D) yolk sac

(E) eggshell

Directions: Each group of questions consists of five lettered headings followed by a list of numbered phrases or sentences. For each numbered phrase or sentence, select the one heading that is most closely related to it and fill in the corresponding oval on the answer sheet. Each heading may be used once, more than once, or not at all in each group.

Questions 9–12

(A) Zygote

(B) Blastocoel

(C) Morula

(D) Blastula

(E) Gamete

9. A germ line cell in a male or a female

10. A fluid-filled cavity that forms after several rounds of cell division

11. Results immediately after fertilization of the ovum by a sperm

12. A solid mass of cells produced by cleavage of the zygote