The primitive pit is a depression in the primitive node at the cranial end of the primitive streak. It extends into the notochordal process and forms the notochordal canal. Thus, it is the entrance to the notochordal canal. The pit later forms the opening of the neurenteric canal, which temporarily connects the amniotic cavity with the umbilical vesicle. Splanchnic mesenchymal cells known as angioblasts aggregate to form isolated masses called blood islands , which develop into vascular endothelium and primordial blood cells.
Blood islands form first on the umbilical vesicle, chorion, allantois, and connecting stalk, but they develop in the embryo about 2 days later.
The allantois, a diverticulum of the umbilical vesicle, is a vestigial structure that later becomes the urachus median umbilical ligament in an adult. It serves as a reservoir for excretory products in some species, but it is nonfunctional in human embryos.
However, its blood vessels become the umbilical vessels. The developing notochord and the adjacent paraxial mesoderm are thought to produce inductive substances that stimulate development of the neural plate from the overlying embryonic ectoderm. The notochord forms the basis of the axial skeleton. The vertebrae develop around it, and then it degenerates. In between the vertebrae, the notochord forms the primordium of the nucleus pulposus of the intervertebral disc.
The neural plate is a thickened area of ectoderm that overlies and extends on each side of the notochord. The neural plate invaginates to form a neural groove. In later development, the neural folds meet dorsally and fuse to form the neural tube. The cranial part of the neural tube develops into the brain, and the longer remaining part forms the spinal cord. The notochord and paraxial mesoderm produce inductive substances that stimulate or induce the overlying ectoderm to thicken and form the neural plate.
The primitive streak produces mesoderm, which develops into mesenchyme embryonic connective tissue. Mesenchyme forms a packing tissue around developing organs and develops into connective tissues and muscles.
The blood islands first appear on the walls of the umbilical vesicle, allantois, and connecting stalk. These extraembryonic membranes are derived from the zygote, but they are not part of the embryo. Blood islands form in the embryo about 2 days after they appear on the umbilical vesicle. The amnion encloses the amniotic cavity, forming an amniotic sac. It contains fluid that bathes the embryonic disc, forming its floor. The wall of this sac consists of an inner epithelial layer of cells covered externally by extraembryonic somatic mesoderm.
The neural groove forms as the neural plate invaginates to form a neural fold on each side. The folds later fuse to form the neural tube, the primordium of the central nervous system brain and spinal cord. The ectoderm lateral to the folds, surface ectoderm, gives rise to the epidermis of the skin.
The intraembryonic mesoderm is derived from the primitive streak. The primitive streak produces mesoderm rapidly during the third and fourth weeks. The embryonic ectoderm in the region indicated forms a neural fold. The neural folds soon fuse, converting the neural plate into the neural tube.
The somites are paired cubical masses derived by segmentation of the paraxial mesoderm. The first pair of somites is formed a short distance caudal to the tip of the notochord, and successive somites are progressively formed from paraxial mesoderm.
Most somites appear between days 20 and 30; they give rise to the axial skeleton and its associated musculature. The notochord arises by transformation of the notochordal process. The notochord is a cellular rod that defines the primordial axis of the embryo.
Mesenchymal cells from the somites later surround it and give rise to the mesenchymal bodies of the vertebrae. The notochord within the developing vertebrae later degenerates.
The intraembryonic coelom in the area indicated becomes part of the peritoneal cavity. The coelom appears here as a space within the lateral mesoderm, splitting it into somatic and splanchnic layers. The transverse section is cut through the caudal region of the lateral extensions of the horseshoe-shaped body cavity or coelom. The splanchnopleure is composed of splanchnic mesoderm and endoderm and represents the future wall of the primordial gut. The endoderm gives rise to the epithelium and glands of the digestive tract, and the mesoderm gives rise to its muscular and fibrous elements.
The neural folds are fusing to form the neural tube, the primordium of the brain and spinal cord. These are the same types of questions you are most likely to find on the exam. You can have confidence these questions cover all the major concepts found the in book. This essential tool can make your academic dreams come true. Give yourself the advantage you deserve. This is the real-world practice you need to improve your test-taking skills.
While results may vary, using this study guide wisely is likely to lead to significantly higher scores. Still not sure? Download our FREE sample to give the test bank a test-drive. Download the free sample and see how we can help you get the grades you deserve!
In stock. SKU test-bank-for-the-developing-human-clini Skip to the end of the images gallery. Skip to the beginning of the images gallery.
Download Download df49ece-developinghuman9emoore. Add to Cart. Add to Compare. Buy now, and get busy studying!
0コメント