amniotic fluid severely inhibit lung development. The formula for under-
standing the relationship between urine and amniotic fluid is:
Less urine output = less amniotic fluid; less swallowing = more amniotic
fluid.
22.The answer is a.(Sadler, pp 67–71, 267. Moore and Persaud, Develop-
ing, pp 67–69. Alberts, pp 1086–1087, 1233.)The first stage of neural tube
formation is the induction by notochord and prechordal plate mesoderm of
the neural plate that forms from ectoderm. This is known as primary
induction and is accompanied by molecular changes in cell adhesion mol-
ecules [restriction to neural cell adhesion molecule (N-CAM)]. This first
stage of neural tube development is followed by a reshaping phase, neuru-
lation, and neural tube closure. Endoderm (answer b)is responsible for
the formation of the GI tract and respiratory system. The somatopleuric
mesoderm(answer c)makes important contributions to the skin (dermis)
and nonmuscle portions of the limbs. The splanchnopleuric mesoderm
(answer d)forms the heart and the muscles of the GI tract and urinary sys-
tem. The hypoblast (answer e)is the thin layer of cells ventral to the epi-
blast; it is displaced by the epiblast cells, which form endoderm.
23.The answer is d.(Sadler, pp 143–147.)The muscles of the extremities
form from the somites that are derived from paraxial mesoderm; the bones,
tendons, and connective tissue of the extremities are derived from somato-
pleuric mesoderm. The intermediate mesoderm is the origin of the urogeni-
tal systems and the adrenal cortex (answer a).The adrenal medulla
(answer b)forms from the neural crest. The humerus (answer c)forms
from somatopleuric mesoderm, but the muscles attached to it are of somite
origin. The masseter (answer e)is a muscle of mastication formed from the
first branchial arch and innervated by branchial visceral efferent (special vis-
ceral efferent) fibers from the nucleus ambiguus compared with the general
somatic efferent innervation of the biceps and other muscles, not of branchial
arch origin.
24.The answer is a.(Sadler, pp 300–303. Moore and Persaud, Developing,
pp 432, 448–450, 473.)The cerebral cortex forms from the telencephalon.
The cortex develops in waves of proliferation, forming layers I to VI with
the innermost layers forming first and the more superficial layers later. The
Embryology: Early and General Answers 91