Early Development of the Embryo

After fertilization, the zygote undergoes a rapid transformation into a ball of cells and then into a trilaminar disc with three distinct layers: endoderm, mesoderm, and ectoderm. The endoderm becomes the lining of the gastrointestinal tract, as well as the liver and pancreatic glandular tissue. The ectoderm gives rise to the epidermis and nervous system. Most other tissue is derived from the mesoderm.

The mesodermal middle layer of this disc develops a lateral cleft in connection with the yolk sac1 (Fig. 2-1). Eventually, the lateral margins move ventrally and medially to encompass the yolk sac (Fig. 2-2). This incorporates the intraembryonic coelom to form a tube within a tube. The outer tube is the body cavity while the inner tube is the primitive gastrointestinal tract. The inner tube maintains a posterior attachment to the body cavity via a dorsal mesentery. Most of its ventral attachment involutes, except at the level of the distal foregut.2

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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