The Elements of Embryology
by Michael Foster (9780217384162)

Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER II. A BRIEF SUMMARY OF THE WHOLE HISTORY OF INCUBATION. Step by step the simple two-layered blastoderm described in the previous chapter is converted into the complex organism of the chick. The details of the many changes through which this end is reached will perhaps be rendered more intelligible if we prefix to the special history of them a brief summary of the general course of events from the beginning to the end of incubation. In the first place, it is to be borne in mind that the embryo itself is formed in the area pellucida, and in the area pellucida alone. The area opaca in no part enters directly into the body of the chick; the structures to which it gives rise are to be regarded as appendages, which sooner or later disappear. Germinal layers. The blastoderm at starting consists of two layers. Very soon a third layer makes its appearance between the other two. These three layers, known as the germinal layers, the establishment of which is a fact of fundamental importance in the history of the embryo, are called respectively the upper, middle and lower layers, or epiblast, mesoblast and hypoblast. Ofthese the epiblast and hypoblast constitute the primary layers. Three similar germinal layers are found in the embryos of all vertebrate and most invertebrate forms, and their history is one of the most important parts of comparative embryology. The epiblast gives rise to the epidermis, the central and peripheral parts of the nervous system, and to the most important parts of the organs of special sense. ] The hypoblast is essentially the secretory lajrer, and furnishes the whole epithelial lining of the alimentary tract and its glands, with the exception of part of the mouth and anus which are lined by the epiblast and are spoken of by embryologists ...

USD $20.49

   Dispatch: Immediate - In Stock