Chapter 04 Organogenetic Period: The Fourth to Eighth Weeks

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.