MULTIPLE CHOICE
Directions: Each group of questions below consists of a numbered list of descriptive words or
phrases accompanied by a diagram with
... [Show More] certain parts indicated by letters and a list of lettered
headings. For each numbered word or phrase, select the lettered part or heading that matches it
correctly and then insert the letter in the space to the right of the appropriate number. Sometimes
more than one numbered word or phrase may be correctly matched to the same lettered part or
heading.
1.
_ Notochordal process
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ANS: B
The notochordal process is a rod-shaped structure composed of cells derived from the primitive
node of the primitive streak. It is the primordium of the notochord, a cellular structure that defines
the primordial axis of the embryo.
2.
_ Site of the prechordal plate
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ANS: A
The site of the prechordal plate is indicated in the drawing by a dotted oval to indicate that it is not
visible from the dorsal surface of the embryonic disc. The prechordal plate is a circular area of
thickened embryonic endoderm in the cranial part of the roof of the umbilical vesicle. The
prechordal plate, together with the overlying embryonic ectoderm, later constitutes the
oropharyngeal membrane.
3.
_ Gives rise to most of the embryonic mesoderm
a. A
b. B
c. C
d. D
e. E
a. A
b. B
c. C
d. D
e. E
a. A
b. B
c. C
d. D
ANS: D
The primitive streak, a linear band of epiblast, gives rise to mesoderm mainly during the third
week. The mesoderm extends laterally and becomes continuous with the extraembryonic
mesoderm on the amnion and umbilical vesicle.
4.
_ Primitive pit
ANS: C
The primitive pit is a depression in the primitive node at the cranial end of the primitive streak. It
extends into the notochordal process and forms the notochordal canal. Thus, it is the entrance to
the notochordal canal. The pit later forms the opening of the neurenteric canal, which temporarily
connects the amniotic cavity with the umbilical vesicle.
5. _ Aggregation of angioblasts
ANS: D
Splanchnic mesenchymal cells known as angioblasts aggregate to form isolated masses
called blood islands, which develop into vascular endothelium and primordial blood cells. Blood
islands form first on the umbilical vesicle, chorion, allantois, and connecting stalk, but they develop
in the embryo about 2 days later. [Show Less]