from The American Heritage® Dictionary of the English Language, 4th Edition
- n. A complex sheet of embryonic cells in craniate vertebrates, formed by association of part of the mesoderm with the ectoderm and developing as the internal body wall.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A fold of tissue, in the embryo of a vertebrate, from which the walls of the body and the amnion develop.
from the GNU version of the Collaborative International Dictionary of English
- n. The outer, or parietal, one of the two lamellæ into which the vertebrate blastoderm divides on either side of the notochord, and from which the walls of the body and the amnion are developed. See splanchnopleure.
from The Century Dictionary and Cyclopedia
- n. The outer one of two divisions of the mesoderm of a four-layered germ, the inner one being the splanchnopleure.
Outside the amniotic ectoderm is a thin layer of mesoderm, which is continuous with that of the somatopleure and is connected by the body-stalk with the mesodermal lining of the chorion.
In reptiles, birds, and many mammals the amnion is developed in the following manner: At the point of constriction where the primitive digestive tube of the embryo joins the yolk-sac a reflection or folding upward of the somatopleure takes place.
The lateral leaves of somatopleure then grow round on each side, and, meeting on the ventral aspect of the allantois, enclose the vitelline duct and vessels, together with a part of the extra-embryonic celom; the latter is ultimately obliterated.
By the forward growth and flexure of the head the pericardial area and the anterior portions of the primitive aortæ are folded backward on the ventral aspect of the fore-gut, and the original relation of the somatopleure and splanchnopleure layers of the pericardial area is reversed.
The lateral mesoderm splits into two layers, an outer or somatic, which becomes applied to the inner surface of the ectoderm, and with it forms the somatopleure; and an inner or splanchnic, which adheres to the entoderm, and with it forms the splanchnopleure (Fig. 16).