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FIGURE 51.23
Hormonal secretion by the placenta. The placenta secretes human chorionic gonadotropin (hCG), which peaks in the second month and then declines. After five weeks, it secretes increasing amounts of estradiol and progesterone.
The high levels of estradiol and progesterone secreted by the placenta prevent ovulation and thus formation of any additional embryos during pregnancy. What would be the expected effect of these high hormone levels in the absence of pregnancy?
Answer: High levels of estradiol and progesterone in the absence of pregnancy would still affect the body in the same way. High levels of both hormones would inhibit the release of FSH and LH, thereby preventing ovulation. This is how birth control pills work. The pills contain synthetic forms of either both estradiol and progesterone or just progesterone. The high levels of these hormones in the pill trick the body into thinking that it is pregnant and so the body does not ovulate.

Self Test

1). An egg surrounded by a granulosa layer is most likely from a
    a). sea urchin.
    b). human.
    c). frog.
    d). fruit fly.
Answer: b

2). The first thing a sea urchin sperm encounters when it makes contact with the egg is the
    a). plasma membrane.
    b). vitelline membrane.
    c). jelly coat.
    d). zona pellucida.
Answer: c

3). Holoblastic cleavage results in
    a). formation of a symmetrical blastula composed of cells of approximately equal size.
    b). formation of an asymmetrical blastula composed of cells of approximately unequal size.
    c). cell division of only the cells near the animal pole.
    d). cell division of only the cells near the vegetal pole.
Answer: a

4). If the trophoblast layer failed to form in a mammalian embryo, which of the following structures would not develop?
    a). the blastopore
    b). the inner cell mass
    c). the archenteron
    d). the fetal placenta
Answer: d

5). Which forms first during gastrulation of a frog embryo?
    a). mouth
    b). anus
    c). dorsal lip
    d). gut
Answer: b

6). Gastrulation in mammals and birds is similar in that
    a). cells migrate over the dorsal lip to generate the archenteron in the anterior hemisphere of the embryo.
    b). cells migrate inward from the upper layer of the blastodisc to form the mesoderm.
    c). cells migrate outward from the upper layer of the blastodisc to form the mesoderm.
    d). cells migrate inward from the lower layer of the blastodisc to form the mesoderm.
Answer: b

7). Neural crest cells break off from the ______________ and later move to the sides of the developing embryo.
    a). placodes
    b). ectoderm
    c). notochord
    d). neural tube
Answer: d

8). The cells of the Spemann organizer are responsible for
    a). notochord development.
    b). cell transplantation.
    c). dorsal lip formation.
    d). development of the eye through an induction mechanism.
Answer: a

9). All of the following structures are derived from the mesoderm except
    a). muscles.
    b). the Liver.
    c). gonads.
    d). blood vessels.
Answer: b

10). Most fetal growth occurs in
    a). the first trimester.
    b). the second trimester.
    c). the third trimester.
    d). the postnatal period.
Answer: c

Test Your Visual Understanding

1). In this picture of a newly fertilized frog embryo, where would you predict that the sperm entered the egg? Draw a sperm at the approximate location of sperm entry.
Answer: The gray crescent forms on the side of the egg opposite from the point of penetration by the sperm. You should have drawn in a sperm in a location similar to that shown in figure 51.4.

Apply Your Knowledge

1). You are preparing for a debate on the use of embryonic stem cells in research. What are the pros and cons of this practice?
Answer: There is no one correct answer to this question.

2). Comparative embryology of vertebrates has led to the phrase ontogeny recapitulates phylogeny. This biogenic law, although not strictly true, supports evolution. Why?
Answer: The phrase ontogeny recapitulates phylogeny implies that embryonic development exhibits a progression of changes indicative of evolutionary origin. However, it is important to realize that embryonic stages are not equivalent to adult ancestral forms. Rather, early developmental stages of an embryo often reflect the embryonic stages of its evolutionary ancestors. This biogenic law supports evolution because the evolution of organisms is essentially "descent with modification" and dependent on changes in the developmental program. In this way, evolution is very closely related to development. Early patterns of development in chordate groups share very similar features, suggesting that these patterns have been built up in incremental steps over the evolutionary history of those groups.

3). What would happen if a mammalian embryo failed to produce sufficient amounts of human chorionic gonadotropin (hCG) in early pregnancy?
Answer: The pregnancy would terminate because hCG is required for production of progesterone and estrogen by the corpus luteum in the first trimester. Progesterone is particularly important for the maintenance of a pregnancy. Without hCG, the corpus luteum would degenerate, and menstruation would occur as in a normal menstrual cycle.

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