The given illustration shows a dorsal view of a human embryo. In the embryo, neurulation is well underway, and several distinct somites are present on each side of the neural tube. The development of
the embryo depicted in this illustration is typical of which of the
following embryonic periods?
(A) Week 1
(B) Week 2
(C) Week 3
(D) Week 4
(E) Week 5
(A) Week 1
(B) Week 2
(C) Week 3
(D) Week 4
(E) Week 5
The answer is D: Week 4.
The illustration shows a dorsal view
of a human embryo at approximately day 24 (during the 4th
week) of development.
Week 4 is characterized by most of
neurulation, much of somite differentiation, the appearance of
the pharyngeal (branchial) apparatus, and the appearance of the
upper limb bud.
Neurulation (the process of formation of the
neural tube) begins late in week 3 with formation of the neural
groove and neural folds.
However, most of the process, including
formation and completion of the neural tube, occursduring
week 4.
Likewise, segmentation of the paraxial mesoderm into
somitomeres and somites begins late in week 3, with 4
to 7 somites formed at day 21. That number increases to 26 to
29 during week 4. Because somites appear at a very specific rate,
their number can be used to determine the age of the embryo
with great accuracy during weeks 3 to 5.
This illustration shows
nearly complete formation of the neural tube plus 17
defi ned somites, indicating the embryo is well into week 4.
The
pharyngeal arches and upper limb buds are not seen here, as
they first appear later in the 4th week of embryonic development.
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