Download 1 Sample Canadian DAT Reading Comprehension

Sample Canadian DAT Reading Comprehension Test
Embryogenesis
above it is the dorsal lip of the blastopore,
derived from the crescent. The cells of the
animal hemisphere move inward more
rapidly than the larger, fewer yolky cells of
the vegetal hemisphere. The slit extends
laterally and downward and eventually
forms a ring of involuting tissue, the
blastopore, which moves over the yolkladen vegetal hemisphere and increasingly
encloses it. This process of overgrowth is
called epiboly.
The starting point for the production of a
new individual by sexual reproduction is
the fertilized egg, or zygote. Repeated
mitotic divisions result in many cells that
differentiate to form the tissues and organs
of the developing individual or embryo.
Soon after an egg is fertilized, the singlecelled zygote becomes two cells, the two
divide into four, and so on. This process of
cleavage partitions the egg substance into
an increasing number of smaller cells, or
blastomeres, each with an equal number of
chromosomes. The cleavage divisions are
unusual because they are not separated by
periods of cellular growth; the zygote mass
is the same at the end of cleavage as at the
start. The size distribution of the cells and
the kind of division are a function of the
amount and distribution of the yolk.
Cleavage is termed holoblastic when an
entire egg cell divides, as in the frog, and
meroblastic when only part of the cell
divides, as in the chick.
Once inside the embryo, the involuting
cells move away from the blastopore and
form the walls of an enlarging chamber,
the archenteron (or gastrocoel), which is
the cavity of the primitive gut. The
infolding process is called invagination. As
the archenteron enlarges, the blastocoel is
gradually obliterated. This is now called
the gastrula. Epiboly and invagination are
accomplished without change in the
overall mass of the embryo, indicating that
growth plays no part in the process of
gastrulation. When complete, the gastrula
consists of an outer layer of ectoderm from
cells of the animal hemisphere, and inner
layer of endoderm called entoderm from
cells of the vegetal hemisphere, and
between these a third layer, mesoderm.
Since the latter at this stage includes the
presumptive notochord, it may be called
chordamesoderm.
As holoblastic cleavage occurs, as in
amphibians, the cells become arranged in
the form of a hollow ball, or blastula,
within which a central cavity, the
blastocoels, appears. Two major regions
are evident, an upper animal hemisphere,
or pole of small dark cells with little yolk,
and an opposite vegetal hemisphere below,
of larger, pale-colored cells rich in yolk
granules. Between them is a marginal zone
of medium-sized cells.
Most of the mesoderm invaginates by
rolling over the lateral and ventral lips of
the blastopore. However, the portion
giving rise to the notochord moves inward
over the dorsal lip and is preceded by the
prechordal plate mesoderm of the head.
These are the germ layers from which
various tissues and organs will form. The
ectoderm will produce the external
covering of the body, the nervous system,
and the sense organs; the endoderm
As cells continue to proliferate, an
inrolling or involution begins directly
below the center of the gray crescent, a
lightly pigmented area of the egg cortex
important in guiding early stages of
development. A slit forms, and the fold
1 provides the lining of the digestive tract, its
glands, and associated structure; and the
mesoderm gives rise to the supportive
tissues, muscles, lining of the body cavity
and other parts.
forebrain and stimulates the ectoderm on
the side of the head region to form a
thickened lens vesicle that subsequently
produces the lens of the eye. Meanwhile,
the outer surface of each optic vesicle
becomes concave by invagination and
forms the retina.
At the end of gastrulation, when all the
endoderm is inside, the original egg axis
has rotated about 90 degrees. The former
lower end of the axis then at the completed
blastopore, marking the posterior end of
the future animal. The chordamesoderm
cells indicate the dorsal region; and shortly
after gastrulation the paired neural folds on
the surface, forward from the blastopore,
provide an external indication of the dorsal
surface.
The endoderm of the primitive gut
becomes the inner lining of the digestive
tract. Anteriorly, at the future pharynx,
three outpocketings of the tract on either
side meet three corresponding inpocketings
from the side of the neck; these break
through to form the gill slits. A single
ventral outpocket, behind the pharynx,
forms the liver bud that becomes the liver
and bile duct. An inpocketing of ectoderm
called stomodeum forms ventrally on the
head region, and a similar one called
proctodeum at the posterior end. In later
embryonic life these break through to join
the endoderm of the digestive tract, the
stomodeum becoming the mouth cavity
and the proctodeum becoming the anal
canal, both lined by ectoderm. During
larval life a ventral outpocket of the
pharynx grows posteriorly and divides into
two lobes. The anterior part gives rise to
the larynx and trachea and the lobes to the
lungs.
After gastrulation, major differentiation of
the embryo begins. From the three germ
layers there are outpocketings,
inpocketings, thickenings, divisions and
other changes that lead to the
establishment of the organs and organ
systems. The nervous system starts
dorsally as a pair of neural folds. The
ectoderm between these sinks down and
the folds come together to form a neural
tube, enlarged at the anterior end to
become the brain. On either side, between
the neural tube and ectoderm, a line of
cells forms the neural crests that will
produce the dorsal or sensory roots of
spinal nerves to grow into the cord. Motor
roots later grow out ventrally from the
cord. The neural crests also contribute
sympathetic ganglia, the Schwann cells of
nerve fibers, pigment cells, and important
cartilage elements of the brachial complex.
During gastrulation, the mesoderm grows
inward and penetrates between the
ectoderm and endoderm. Cells in its
middorsal part become arranged as a solid
rod, the notochord, between the nerve tube
and primitive gut, to serve as a supporting
body axis. Prospective mesoderm at either
side of the notochord grows down as a
curved plate between the ectoderm and
endoderm, and the two meet ventrally
under the yolk-laden cells. The thin lower
part of each plate called hypomere splits
into two layers. The outer is applied to the
ectoderm and becomes the parietal
peritoneum, the inner surrounds the gut to
The early brain has three primary vesicles,
the forebrain, midbrain and hindbrain. The
forebrain produces the cerebral
hemispheres and diencephalon, and from
the hindbrain the cerebellum and medulla
oblongata are derived. A rounded optic
vesicle grows laterally on either side of the
2 make the visceral peritoneum and smooth
muscle of the gut, and the space between
the layers is the body cavity, or coelom.
The upper most mesoderm called epimere
at either side of the nerve tube and
notochord forms a lengthwise series of
segmental blocks or somites. Each somite
differentiates into three parts: a thin outer
part called dermatome becomes the dermis
of the skin, a thick inner part called
myotome gives rise to voluntary muscles,
and nearest the notochord, a scattering of
cells called sclerotome grow about the
neural tube and notochord to form the
vertebrae or axial skeleton. Between the
ventral plates and the somites a third
portion called mesomere is the forerunner
of the excretory system and parts of the
reproductive system.
2. The mesoderm gives rise to which
part(s) of the body?
A.
B.
C.
D.
E.
3. Kevin, a 20 year-old male, came
into the veterinarian’s clinic to
consult something regarding his
dog named Troy. According to
Kevin, Troy seemed “not to hear”
anything since his puppy days.
Kevin also reported that he also had
a hard time calling Troy and that
Troy seldom barks. After some
tests, the veterinarian said that Troy
is deaf and that Troy did not
develop his sense organ for
hearing. Defect in which germ
layer caused Troy’s deafness?
1. TRUE about embryogenesis:
A. The process of cleavage
partitions the egg substance into
an increasing number of smaller
cells with smaller number of
chromosomes.
B. At the end of gastrulation, when
all the endoderm is inside, the
original egg has rotated about
90 degrees.
C. Epiboly and invagination
causes a change in the overall
mass of the embryo.
D. Amphibians, like frogs,
undergo meroblastic cleavage.
E. All of the above
Muscles
Cartilage elements
Nervous system
Mouth
None of the above
A.
B.
C.
D.
E.
3 Endoderm
Mesoderm
Entoderm
Ectoderm
Chordamesoderm
Answer key:
1. B
2. A
3. D
4