1. Your cousin just had twins. She tells you that twinning occurs when two sperm
fertilize the same egg. You reply that—
a. Yes, she is right, that is the most common source of twinning.
b. No, only one sperm survives passage through the uterine cervix, so two
sperm are never present at fertilization.
c. No, cortical granules are used to prevent additional sperm penetration.
d. No, twinning occurs when unfertilized eggs divide spontaneously and thus
is parthenogenic in nature.
The correct answer is c—
A. Answer a is incorrect. Fertilization by two sperm would create a triploid zygote
instead of a diploid one. There would be too much genetic material for a single
embryo but not enough to generate two.
The correct answer is c—
B. Answer b is incorrect. Although the majority of sperm die on their way to the
oviducts, many of the millions released during ejaculation are still viable after
they pass through the cervix.
The correct answer is c—No, cortical granules are used to prevent additional sperm
penetration.
C. Answer c is correct. Cortical granules are one the mechanisms used by eggs to
prevent polyspermy.
The correct answer is c—
D. Answer d is incorrect. At least in humans, parthenogenesis (development without
fertilization) does not occur.
2. Prior to fertilization, the mammalian egg completes the second meiotic division.
a. This statement is true, meiosis II is actually completed before ovulation.
b. This statement is false because eggs don’t undergo meiosis; they are
descended by cloning from a haploid germ cell line.
c. This statement is false because mammalian eggs don’t begin meiosis until
after fertilization.
d. This statement is false because mammalian eggs do not release the second
polar body until the sperm has entered the egg.
The correct answer is d—
A. Answer a is incorrect. Mammalian eggs do not release the second polar body
(signaling the end of meiosis II) until after fertilization occurs.
The correct answer is d—
B. Answer b is incorrect. The process of oogenesis involves meiosis.
The correct answer is d—
C. Answer c is incorrect. Eggs begin meiosis prior to ovulation .
The correct answer is d—This statement is false because mammalian eggs do not release
the second polar body until the sperm has entered the egg.
D. Answer d is correct. Meiosis II ends after fertilization.
3. Which of the following events occur immediately after fertilization?
a. Egg activation
b. Polyspermy defense
c. Cytoplasm changes
d. All of the above
The correct answer is d—
A. Answer a is incorrect. It is only one of three correct answers.
The correct answer is d—
B. Answer b is incorrect. It is only one of three correct answers.
The correct answer is d—
C. Answer c is incorrect. It is only one of three correct answers.
The correct answer is d—All of the above
D. Answer d is correct. All of these choices occur immediately after fertilization and
could be used for your project.
4. Suppose that a burst of electromagnetic radiation were to strike the blastomeres of
only the animal pole, which of the following would be most likely to occur?
a. A change or mutation relevant to the epidermis or skin
b. A switching of the internal organs so that reverse orientation (left/right)
occurs along the midline of the body
c. The migration of the nervous system to form outside of the body
d. Failure of the reproductive system to develop
The correct answer is a—A change or mutation relevant to the epidermis or skin
A. Answer a is correct. The animal pole gives rise to structures on the outer layer of
the body.
The correct answer is a—
B. Answer b is incorrect. The animal pole does not give rise to structures in the
interior of the body and therefore this is unlikely to occur.
The correct answer is a—
C. Answer c is incorrect. The animal pole does not give rise to structures in the
interior of the body and therefore this is unlikely to occur.
The correct answer is a—
D. Answer d is incorrect. The animal pole does not give rise to structures in the
interior of the body and therefore this is unlikely to occur.
5. Which of the following plays the greatest role in determining how cytoplasmic
division occurs during cleavage?
a. Number of chromosomes
b. Amount of yolk
c. Orientation of the vegetal pole
d. Sex of the zygote
The correct answer is b—
A. Answer a is incorrect. The number of chromosomes within a zygote does not
determine the division pattern.
The correct answer is b—Amount of yolk
B. Answer b is correct. The amount of yolk (holoblastic, meroblastic) determines the
pattern of division.
The correct answer is b—
C. Answer c is incorrect. The orientation of the vegetal pole does not determine the
division pattern.
The correct answer is b—
D. Answer d is incorrect. The sex of the zygote does not determine the division
pattern.
6. Gastrulation is a critical event during development. Why?
a. Gastrulation converts a hollow ball of cells into a bilaterally symmetrical
structure.
b. Gastrulation causes the formation of a primitive digestive tract.
c. Gastrulation causes the blastula to develop a dorsal–ventral axis.
d. All of the above.
The correct answer is d—
A. Answer a is incorrect. It is only one of three correct answers.
The correct answer is d—
B. Answer b is incorrect. It is only one of three correct answers.
The correct answer is d—
C. Answer c is incorrect. It is only one of three correct answers.
The correct answer is d—All of the above.
D. Answer d is correct. It incorporates all of the correct answers.
7. Gastrulation in a mammal would be most similar to gastrulation in—
a.
b.
c.
d.
a gecko
a tuna
an eagle
no other species; mammalian gastrulation is unique
The correct answer is c—
A. Answer a is incorrect. Mammalian gastrulation is most similar to birds.
The correct answer is c—
B. Answer b is incorrect. Mammalian gastrulation is most similar to birds.
The correct answer is c—an eagle
C. Answer c is correct. Mammalian gastrulation is most similar to birds.
The correct answer is c—
D. Answer d is incorrect. All vertebrates share similarities in developmental
processes; ours happen to be most similar to those of birds.
8. Somites—
a. begin forming at the tail end of the embryo and then move forward in
wavelike fashion
b. are derived from endoderm
c. develop into only one type of tissue per somite
d. may vary in number from one species to the next
The correct answer is d—
A. Answer a is incorrect. Somites actually form from head toward tail.
The correct answer is d—
B. Answer b is incorrect. Somites are derived from mesoderm.
The correct answer is d—
C. Answer c is incorrect. Somites differentiate into different types of connective
tissues including bone and muscle.
The correct answer is d—may vary in number from one species to the next
D. Answer d is correct. The number of somites varies from one species to another.
9. Of the processes following, which occurs last?
a. Cleavage
b. Neurulation
c. Gastrulation
d. Fertilization
The correct answer is b—
A. Answer a is incorrect. Cleavage occurs after fertilization and before gastrulation.
The correct answer is b—Neurulation
B. Answer b is correct. Neurulation is the last of these stages to occur.
The correct answer is b—
C. Answer c is incorrect. Gastrulation occurs before neurulation.
The correct answer is b—
D. Answer d is incorrect. Of the phases listed, fertilization would occur first.
10. Which of the following would qualify as a secondary induction?
a. The formation of the lens of the eye due to induction by the neural
ectoderm
b. Differentiation during neurulation by the dorsal ectoderm and mesoderm
c. Both of these
d. Neither of these
The correct answer is a—The formation of the lens of the eye due to induction by the
neural ectoderm
A. Answer a is correct. The formation of the lens of the eye is an example of
secondary induction.
The correct answer is a—
B. Answer b is incorrect. Differentiation by the dorsal ectoderm and mesoderm is an
example of primary induction.
The correct answer is a—
C. Answer c is incorrect. Answer a is correct.
The correct answer is a—
D. Answer d is incorrect. Answer a is correct.
11. Your Aunt Ida thinks that babies can stimulate the onset of their own labor. You
tell her that—
a. no, among mammals the onset of labor has been most closely linked to a
change in the phases of the moon
b. no, it is the mother’s circadian clock that determines the onset of labor
c. yes, body weight determines the onset of labor
d. yes, changes in fetal hormone levels can determine the onset of labor
The correct answer is d—
A. Answer a is incorrect. There is no scientific evidence that certain phases of the
moon cause labor.
The correct answer is d—
B. Answer b is incorrect. There is no scientific evidence that the mother’s circadian
clock sets the time for the onset of labor.
The correct answer is d—
C. Answer c is incorrect. Although full-term fetuses are larger than preterm ones,
weight at birth is sufficiently varied to be an unlikely cause of the onset of labor.
The correct answer is d—yes, changes in fetal hormone levels can determine the onset of
labor
D. Answer d is correct. Research indicates that changes in the output of fetal
hormones may cause the onset of labor. Therefore, your aunt may actually be
right (as usual).
12. Drug or alcohol exposure during which of the following stages is most likely to
have a profound effect on the neural development of the fetus?
a. Preimplantation
b. First trimester
c. Second trimester
d. Third trimester
The correct answer is b—
A. Answer a is incorrect. Prior to implantation the zygote is not exposed to the
mother’s blood and therefore this is not the stage most likely to be affected by
toxin exposure.
The correct answer is b—First trimester
B. Answer b is correct. The first trimester includes the formation of the
cardiovascular and nervous systems. The development of either or both of these
can be affected by toxic exposure.
The correct answer is b—
C. Answer c is incorrect. The most sensitive period for the embryo is during the first
trimester.
The correct answer is b—
D. Answer d is incorrect. The most sensitive period for the embryo is during the first
trimester.
13. Axis formation in amniotic embryos can be affected by—
a. mutation of the dorsal lip of the blastopore
b. mutation of the primitive streak structure
c. both of these
d. neither of these
The correct answer is c—
A. Answer a is incorrect. Both the dorsal lip and the primitive streak play roles in
axis formation.
The correct answer is c—
B. Answer b is incorrect. Both the dorsal lip and the primitive streak play roles in
axis formation.
The correct answer is c—both of these
C. Answer c is correct. Both the dorsal lip and the primitive streak play roles in axis
formation.
The correct answer is c—
D. Answer d is incorrect. The dorsal lip and the primitive streak play roles in axis
formation.
14. How would a given cell know whether it was to become skin or muscle tissue?
a. The level of Sonic hedgehog molecules it is exposed to
b. The size of nearby cells
c. The number of chromosomes within its nucleus
d. The age of the embryo when the cell differentiates
The correct answer is a—The level of Sonic hedgehog molecules it is exposed to
A. Answer a is correct. The relative amount of Sonic hedgehog molecules will
determine what type of tissue a cell will become.
The correct answer is a—
B. Answer b is incorrect. The size of the nearby cells would not determine what type
of tissue a cell would become.
The correct answer is a—
C. Answer c is incorrect. The number of chromosomes is the same within each of the
cells within the embryo and so number of chromosomes will not determine what
type of tissue a cell will become.
The correct answer is a—
D. Answer d is incorrect. The age of the embryo will affect all cells equally and
therefore will not determine what type of tissue a cell will become.
Challenge Questions
1. Suppose you discover a new species whose development mechanisms have not
been documented before. How could you determine at what stage the cell fate is
determined?
Answer—By starting with a series of embryos at various stages, you could try removing
cells at each stage. Embryos that failed to compensate for the removal (evidenced by
missing structures at maturity) would be those that lost cells after they had become
committed; that is, when their fate has been determined.
2. You look up from your studying to see your dog, Fifi, acting silly again. Using
this as a teachable moment, compare and contrast the homeoboxes in your dog
and the fruit fly she just ate.
Answer—Homeoboxes are sequences of conserved genes that play crucial roles in
development of both mammals (Fifi) and Drosophila (the fruit fly). In fact, we know that
they are more similar than dissimilar; research has demonstrated that both groups use the
same transcription factors during organogenesis. The major difference between them is
in the genes that are transcribed. Homeoboxes in mammals turn on genes that cause the
development of mammalian structures, and those in insects would generate insect
structures.
3. Why doesn’t a woman menstruate while she is pregnant?
Answer—After fertilization, the zygote produces hCG, which inhibits menstruation and
maintains the corpus luteum. At 10 weeks gestation, the placenta stops releasing hCG
but it does continue to release estradiol and progesterone, which maintain the uterine
lining and inhibit the pituitary production of FSH and LH. Without FSH and LH no
ovulation and no menstruation occur.
4. Spemann and Mangold were able to demonstrate that some cells act as
“organizers” during development. What types of cells did they use? How did
they determine that these cells were organizers?
Answer—Spemann and Mangold removed cells from the dorsal lip of one amphibian
embryo and transplanted them to a different location on a second embryo. The
transplanted cells caused cells that would normally form skin and belly to instead form
somites and the structures associated with the dorsal area. Because of this and because
the secondary dorsal structures contained both host and transplanted cells, Spemann and
Mangold concluded that the transplanted cells acted as organizers for dorsal
development.