Organogenetic Period: The Fourth to Eighth Weeks

MULTIPLE CHOICE

Directions: Each group of questions below consists of a numbered list of descriptive words or phrases accompanied by a diagram with certain parts indicated by letters or by a list of lettered headings. For each numbered word or phrase, select the letter or heading that matches it correctly. 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.        

________ Hyoid arch

a.
A
b.
B
c.
C
d.
D
e.
E

ANS:   D

The second or hyoid pharyngeal arch is recognizable early in the fourth week. As its name implies, this arch gives rise to part of the hyoid bone. Mesenchyme in this arch also gives rise to the muscles of facial expression and to various skeletal structures.

        2.        

________ Gives rise to the mandible

a.
A
b.
B
c.
C
d.
D
e.
E

ANS:   C

The mandibular prominences of the first pair of pharyngeal arches give rise to the lower jaw or mandible. Rostral extensions of the first arch—maxillary prominences—give rise to the upper jaw or maxilla.

        3.        

________ Forms from the paraxial mesoderm

a.
A
b.
B
c.
C
d.
D
e.
E

ANS:   E

The somites form by differentiation and division of longitudinal columns of paraxial mesoderm into cubical segments. The somites give rise to most of the axial skeleton, the associated musculature, and the dermis of the skin.

        4.        

________ Neuropore

a.
A
b.
B
c.
C
d.
D
e.
E

ANS:   B

The caudal neuropore is indicated. The neuropores (rostral and caudal) normally close by the end of the fourth week. The rostral neuropore closes on day 25 or 26, and the caudal neuropore usually closes about 2 days later. Defective closure of the caudal neuropore gives rise to a severe anomaly—spina bifida.

        5.        

________ Optic groove in neural fold

a.
A
b.
B
c.
C
d.
D
e.
E

ANS:   A

The pointer indicates the optic groove in a neural fold. This sulcus is the primordium of the optic vesicle, an outgrowth of the forebrain vesicle.

        6.   ________ Forms the major part of the diaphragm

A.
Limb bud
B.
Neuropore
C.
Septum transversum
D.
Cervical sinus
E.
Stomodeum

ANS:   C

The septum transversum is a mass of mesoderm that appears cranial to the developing heart. After folding of the embryo, it lies caudal to the heart, where it forms part of the primordial diaphragm. Later it develops into the central tendon of the diaphragm.

        7.   ________ Primordial mouth

A.
Limb bud
B.
Neuropore
C.
Septum transversum
D.
Cervical sinus
E.
Stomodeum

ANS:   E

The stomodeum or primordial mouth is an ectodermal depression that develops during the fourth week as the head folds. Initially, it is separated from the primordial pharynx (cranial part of foregut) by the oropharyngeal membrane. By the end of the fourth week, this membrane ruptures, bringing the mouth into communication with the foregut.

        8.   ________ Outgrowth of the ventrolateral body wall

A.
Limb bud
B.
Neuropore
C.
Septum transversum
D.
Cervical sinus
E.
Stomodeum

ANS:   A

The limb buds form on the ventrolateral body wall during the fourth week. The upper limb buds usually appear on day 26, and the lower limb buds are visible about 2 days later.

        9.   ________ Closes during the fourth week

A.
Limb bud
B.
Neuropore
C.
Septum transversum
D.
Cervical sinus
E.
Stomodeum

ANS:   B

The neuropores close during the fourth week. The rostral neuropore closes on day 25 or 26, and the caudal neuropore closes about 2 days later. Defects of closure of the neuropores give rise to congenital anomalies of the central nervous system (e.g., spina bifida).

      10.   ________ Ectodermal depression in the neck

A.
Limb bud
B.
Neuropore
C.
Septum transversum
D.
Cervical sinus
E.
Stomodeum

ANS:   D

The cervical sinus is a depression in the surface ectoderm on each side of the future neck. It forms when the second pharyngeal arch overgrows the third and fourth arches. The second and third arches come to lie at the bottom of a pit—the cervical sinus. It is visible externally only during the fifth week.

      11.  

______ Part of the embryonic coelom

a.
A
b.
B
c.
C
d.
D
e.
E

ANS:   D

The septum transversum is a mass of mesoderm that is first recognizable cranial to the pericardial coelom. After folding, it lies caudal to the heart and pericardial coelom. The septum transversum gives rise to the central tendon of the definitive diaphragm.

      12.  

______ Oropharyngeal membrane

a.
A
b.
B
c.
C
d.
D
e.
E

ANS:   A

The oropharyngeal membrane develops during the third week as the prechordal plate fuses with the overlying embryonic ectoderm. This membrane ruptures during the fourth week.

      13.  

______ Separates the amniotic cavity from the foregut

a.
A
b.
B
c.
C
d.
D
e.
E

ANS:   A

The oropharyngeal membrane separates the amniotic cavity from the foregut. Amniotic fluid may enter the primordial mouth or stomodeum, but it cannot pass into the primordial pharynx until the oropharyngeal membrane ruptures.

      14.  

______ Gives rise to a major part of the diaphragm

a.
A
b.
B
c.
C
d.
D
e.
E

ANS:   D

The septum transversum is the first recognizable part of the developing diaphragm. It appears cranially at the end of the third week, when the pericardial coelom forms. The septum transversum later forms the central tendon of the diaphragm.

      15.  

______ Cloaca

a.
A
b.
B
c.
C
d.
D
e.
E

ANS:   B

The cloaca is the dilated caudal part of the hindgut; the allantois enters it ventrally. The cloaca is separated from the amniotic cavity by the cloacal membrane.

      16.  

______ Produces embryonic mesoderm

a.
A
b.
B
c.
C
d.
D
e.
E

ANS:   C

The primitive streak appears during the early part of the third week and produces mesoderm rapidly until the end of the fourth week. Thereafter, mesoderm production from this source slows down.

      17.  

______ Separates the amniotic cavity from the hindgut

a.
A
b.
B
c.
C
d.
D
e.
E

ANS:   E

The cloacal membrane separates the amniotic cavity from the cloacal region of the hindgut. This membrane consists of fused layers of embryonic ectoderm and endoderm. It is later divided into anal and urogenital membranes during the seventh week; these membranes soon rupture.

      18.  

______ Amnion

a.
A
b.
B
c.
C
d.
D
e.
E

ANS:   D

The amnion forms during the second week. It is attached to the margins of the embryonic disc and forms the wall of the amniotic sac.

      19.  

______ Site of blood formation in the early embryo

a.
A
b.
B
c.
C
d.
D
e.
E

ANS:   A

Blood begins to form in the mesenchyme around the allantois and umbilical vesicle during the third week. These are the only sites of blood formation until blood begins to form in the liver during the sixth week.

      20.  

______ Second pharyngeal arch

a.
A
b.
B
c.
C
d.
D
e.
E

ANS:   A

The second pharyngeal arch appears at the beginning of the fourth week. The embryo shown is about five weeks old and has four pairs of pharyngeal arches; the fourth pharyngeal arch is smaller and not clearly delineated. The second pharyngeal arch overgrows the third and fourth arches, forming a transitory ectoderm-lined cervical sinus that becomes obliterated.

      21.  

______ Stomodeum

a.
A
b.
B
c.
C
d.
D
e.
E

ANS:   C

An ectoderm-lined depression—the stomodeum—partly contributes to the formation of the oral cavity. It is located at the cranial end of the embryo, bounded by the maxillary and mandibular prominences. The oropharyngeal membrane breaks down at about 26 days, bringing the primordial pharynx into communication with the amniotic cavity.

      22.  

______ Will form the lower jaw

a.
A
b.
B
c.
C
d.
D
e.
E

ANS:   D

The first pharyngeal arch gives rise to both the maxillary and mandibular prominences. The lower jaw is formed from the larger mandibular prominences, which are paired at first, but the medial ends merge and fuse during the fourth week.

      23.  

______ Gives rise to the upper jaw

a.
A
b.
B
c.
C
d.
D
e.
E

ANS:   B

The shorter and smaller maxillary prominences form the lateral boundaries of the stomodeum. The maxillary prominences, which lie just below the overhanging frontonasal prominence, grow medially and merge with the medial nasal prominences to form the upper lip. The upper jaw develops from the maxillary prominences, which also give rise to several facial and cranial bones.

      24.  

______ Defect in its formation results in a severe type of spina bifida

a.
A
b.
B
c.
C
d.
D
e.
E

ANS:   E

Fusion of the neural folds occurs during the fourth week, forming the neural tube. The neural tube gives rise to the brain and spinal cord. Neural tube defects, including a severe type of spina bifida, occur as a result of defective closure of the neural tube.