HUMAN BIOLOGY

350 Chapter 17

summary

section 17.1 The fertilization of an

oocyte by a sperm launches several key

stages in early development.

During cleavage, the fertilized egg

undergoes cell divisions that form the early

multicellular embryo. The destiny of various

cell lines is established in part by the portion of cytoplasm

inherited at this time.

Gastrulation lays out the organizational framework of the

whole body. Endoderm, ectoderm, and mesoderm form; all

the tissues of the adult body will develop from these three

germ layers.

Differentiation is the process during which cells come to

have specific structures and functions.

In morphogenesis, tissues form and become organized into

organs. Tissues and organs continue to mature as the fetus

develops and even after birth.

sections 17.2, 17.3 During the first

week or so after fertilization, cell divisions

and other changes transform the zygote into

a multicellular blastocyst, which attaches to

the mother’s uterus during implantation. The

blastocyst includes the inner cell mass, a small clump of cells

from which the embryo develops.

expLoreon your oWn

section 17.4 Gastrulation and morpho-

genesis shape the body’s basic plan. A key

step is the formation of the neural tube, the

forerunner of the brain and spinal cord, from

ectoderm. The skeleton and most muscles

develop from blocks of cells called somites

that arise from mesoderm. In morphogenesis,

sheets of cells fold and cells migrate to new locations in the

developing embryo.

sections 17.5, 17.6 During implant-

ation, the inner cell mass is transformed into

an embryonic disk. Some of its cells give

rise to four extraembryonic membranes:

the yolk sac, the allantois, the amnion, and

the chorion. Table 17.4 summarizes their

functions. The embryo and its mother exchange nutrients,

gases, and wastes by way of the placenta, a spongy organ

that is a combination of endometrium and extraembryonic

membranes.

section 17.7 The first 8 weeks of

development are the embryonic period;

thereafter the developing individual is

considered a fetus. By the ninth week of

development, the fetus clearly looks human.

Housecats can carry the parasite that causes toxoplasmosis. In an otherwise

healthy person the disease may only produce flulike symptoms, but it is dangerous for a

pregnant woman and her fetus. A mother-to-be may suffer a miscarriage, and if the para-

site infects a fetus it causes birth defects. An infected cat may not appear to be ill, but its

feces will contain infectious cysts. This is why some physicians advise pregnant women to

avoid contact with cats and not to clean sandboxes, take care of housecat “accidents,” or

empty a litterbox.

All that said, toxoplasmosis is not especially common—so is it something a cat-loving

expectant mother should take seriously? To explore this health concern, research the

kinds of birth defects caused by toxoplasmosis and find out what stance (if any) public

health authorities in your community take on this issue. Is the disease more common

in some regions or settings than in others? Can cat owners have their pets tested for

the disease?

Lauren and Homer. Photography by Gary Head

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