Diagnostic ultrasound ( PDFDrive )

CHAPTER 30 The First Trimester 1053
A
B
C
FIG. 30.4 Formation of Secondary Yolk Sac. (A) Approximately 26 days: formation of cavities within extraembryonic mesoderm. These cavities
will enlarge to form extraembryonic coelom. (B) About 27 days and (C) 28 days: formation of secondary yolk sac with extrusion of primary yolk
sac. Extraembryonic coelom will become chorionic cavity. (With permission from Moore KL. The developing human: clinically oriented embryology.
10th ed. Philadelphia: Elsevier; 2016. 1 )
Similarly, the rapidly growing embryonic implantation must be
vascularized by the 3-mm stage that occurs at 5 weeks’
gestation.
During the ith week, the embryo is converted by the process
of gastrulation from a bilaminar disk to a trilaminar disk with
the three primary germ cell layers: ectoderm, mesoderm, and
endoderm. During gastrulation, the primitive streak and notochord
form. he primitive streak gives rise to the mesenchyme,
which forms the connective tissue of the embryo and stromal
components of all glands.
he formation of the neural plate and its closure to form the
neural tube is referred to as neurulation. his process begins
in the ith week in the thoracic region and extends caudally and
cranially, resulting in complete closure by the end of the sixth
week (day 42). Failure of closure of the neural tube results in
neural tube defects.
During the ith week, two cardiac tubes (the primitive heart)
develop from splanchnic mesodermal cells. By the end of the
ith week, these tubes begin to pump into a primitive paired
vascular system. By the end of the ith week, a vascular network
develops in the chorionic villi that connect through the umbilical
arteries and vein to the primitive embryonic vascular network.
Essentially all internal and external structures appear in the
adult form during the embryonic period, which ends at 10