A manual of veterinary physiology / by F. Smith.

Date:
1892
Catalogue details

Licence: Public Domain Mark

Credit: A manual of veterinary physiology / by F. Smith. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    time becomes pear-shaped, and corresponds to the direction of the future embryo. At the posterior part of the germinal area the primitive streak appears, and soon becomes a groove termed the primitive groove. Leaving the ovum for the moment, we must describe the changes which have occurred in the bhistodermic layer which was left on the inside of the vitelline membrane. It is found that this layer divides at first into two, and then into three layers. The outer, called the ectoderm, epiblast, or serous layer, grows the central nervous system and epidermal tissues, including hoof The inner or lower layer is termed the endoblast or hypoblast, and grows the intestinal canal and the glands opening into it. The middle layer, or mesoblast, forms the bloodvessels, and constitutes a part termed the area vasculosa ; it also grows the muscles and skeleton, generative and excretory organs. Returning to the primitive groove, it is found that in front of it is formed another groove, the medullary, which, extending back and opening out, encloses the primitive groove and causes it to disappear. The growth of the medullary groove has given rise to a part termed the area pellucida, surrounded by an opaque surface, termed the area opaca. In the pellucid area is formed the embryo. The medullary groove grows from the epiblast; from either side of the groove rises up a structure termed the lamina dorsales, which, growing over the groove, meet, and thus form a canal termed the neural canal; in this is developed the brain and spinal cord. Underneath the medullary groove is formed from the mesoblast a cord termed the notochord, from and around which are developed the vertebne. From another portion of the mesoblast is formed the pleuro-peritoneal cavity. Two important structures appear at this time, developing from one or other of the layers into which we have seen the blastodermic membrane is divided. By the union of certain layers of the mesoblast with the epiblast a structure termed the somatopleure is formed, and from a layer of the