HUMAN BIOLOGY

Development anD aging 351

section 17.8 During the last 3 months
of gestation (the third trimester), the fetus
grows rapidly and many organs mature.
However, because the fetus exchanges gases
and receives nourishment via its mother’s
bloodstream, its own circulatory system
routes blood flowing to the lungs and liver through temporary
blood vessels.

sections 17.9, 17.10 Birth takes place
approximately 39 weeks after fertilization.
Labor advances through three stages; a baby
is born at the end of stage two, and the
afterbirth (placenta) is expelled in stage three.
At birth, contractions of the uterus expel the
fetus and afterbirth. After delivery, nursing
causes the secretion of hormones that stimulate lactation—the
production and release of milk.
Miscarriage is the spontaneous expulsion of a developing
embryo before 20 weeks of gestation. The term “stillbirth”
refers to the death and delivery of a fetus after 20 weeks.
Poor maternal nutrition, certain infections, and use of drugs
including nicotine, alcohol, and therapeutic drugs may have
serious harmful effects on the mother’s developing fetus.

sections 17.12, 17.13 Human
development can be divided into a prenatal
period before birth, followed by the neonate
(newborn) stage, childhood, adolescence,
and adulthood. The process of aging is called
senescence. As the body ages, changes occur
in the structure and functional efficiency of many organ
systems. These changes are due to multiple factors.

Membrane Function

Yolk sac Source of digestive tube; helps form blood cells

and forerunners of gametes

Allantois Source of umbilical blood vessels and vessels of

the placenta

Amnion Sac of fluid that protects the embryo and keeps

it moist

Chorion Forms part of the placenta; protects the embryo

and the other extraembryonic membranes

Table 17.4 Extraembryonic Membranes

reVieW Questions

1. Define and describe the main features of the following
   developmental stages: fertilization, cleavage, gastrulation.


2. Define cell differentiation and morphogenesis, two
   processes that are critical for development.


3. Summarize the development of an embryo and a fetus.
   When are body parts such as the heart, nervous system,
   and skeleton largely formed?


4. Label the early development stages shown in the following
   diagram.

© Cengage Learning

seLF-Quiz Answers in Appendix VI

1. Development cannot proceed properly unless each of the
   following processes is successfully completed before the
   next begins, starting with.
   a. gamete formation d. gastrulation
   b. fertilization e. organ formation
   c. cleavage f. growth, tissue
   specialization


2. During cleavage, the is converted to a ball of cells,
   which in turn is transformed into the.
   a. zygote; blastocyst c. ovum; embryonic disk
   b. trophoblast; embryonic disk d. blastocyst; embryonic
   disk


3. In the week following implantation, cells of the will
   give rise to the embryo.
   a. blastocyst c. embryonic disk
   b. trophoblast d. zygote

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