The following sections of this BookRags Literature Study Guide is offprint from Gale's For Students Series: Presenting Analysis, Context, and Criticism on Commonly Studied Works: Introduction, Author Biography, Plot Summary, Characters, Themes, Style, Historical Context, Critical Overview, Criticism and Critical Essays, Media Adaptations, Topics for Further Study, Compare & Contrast, What Do I Read Next?, For Further Study, and Sources.
(c)1998-2002; (c)2002 by Gale. Gale is an imprint of The Gale Group, Inc., a division of Thomson Learning, Inc. Gale and Design and Thomson Learning are trademarks used herein under license.
The following sections, if they exist, are offprint from Beacham's Encyclopedia of Popular Fiction: "Social Concerns", "Thematic Overview", "Techniques", "Literary Precedents", "Key Questions", "Related Titles", "Adaptations", "Related Web Sites". (c)1994-2005, by Walton Beacham.
The following sections, if they exist, are offprint from Beacham's Guide to Literature for Young Adults: "About the Author", "Overview", "Setting", "Literary Qualities", "Social Sensitivity", "Topics for Discussion", "Ideas for Reports and Papers". (c)1994-2005, by Walton Beacham.
All other sections in this Literature Study Guide are owned and copyrighted by BookRags, Inc.
Cell division of a fertilized egg results in the formation of a population of presumably equivalent cells. The initially large egg is cleaved into smaller embryonic cells. Cleavage is followed by gastrulation. Gastrulation is the beginning of the process of cell specialization. Gastrulation is an extraordinarily critical stage in development because the cells of the former blastula are rearranged by folding and cell migration to construct an embryo of three layers. The outer cell layer of an amphibian gastrula and the superficial cell layer of a bird, reptile and mammalian primitive streak stage is referred to as ectoderm. Ectoderm, and the other cell layers, known as mesoderm and endoderm, comprise the three primary germ layers of vertebrate embryos.
Embryonic ectoderm differentiates into a diversity of adult cells, tissues and organs. These include the integument, the nervous system, and external sense organs. The ectodermal integument derivatives consist of epidermis, nails, hair, tooth enamel, and glands associated with the skin, such as the mammary, sweat and sebaceous glands. The ectodermal central nervous system is comprised of the brain and spinal cord. Ectoderm also gives rise to spinal nerves, cranial nerves, the autonomic nervous system, and the adrenal medulla. The nervous system contains neurons which are the essential conducting cells. The neurons are supported and maintained by ectodermal neuroglia cells. External sense organs of ectodermal origin include the nose, eyes, and ears. The mucous membranes of the oral cavity and anus are ectodermal in origin.