. Kirkes' handbook of physiology . ristic of animal nature, is, when taken by itself, noproof of it. Cell Differentiation and the Functions of Organized Cells. As weproceed upward in the scale of life from the unicellular organisms, we findanother phenomenon exhibited in the life history of the higher forms, namely,that of development. The one-celled ameba comes into being derived froma previous ameba; it manifests the properties and performs the functions ofits life which have been already enumerated. In the higher organisms it isdifferent. Each, indeed, begins as a single cell, but the cells
. Kirkes' handbook of physiology . ristic of animal nature, is, when taken by itself, noproof of it. Cell Differentiation and the Functions of Organized Cells. As weproceed upward in the scale of life from the unicellular organisms, we findanother phenomenon exhibited in the life history of the higher forms, namely,that of development. The one-celled ameba comes into being derived froma previous ameba; it manifests the properties and performs the functions ofits life which have been already enumerated. In the higher organisms it isdifferent. Each, indeed, begins as a single cell, but the cells which result fromdivision and subdivision do not form so many independent organisms, butadhere in one differentiated community which ultimately forms the complexbut co-ordinated whole, in man the human body. Thus, from the ovum, or germ cell which forms the starting-point of ani-mal life, in a comparatively short time there is formed a complete membraneof cells, polyhedral in shape from mutual pressure, called the Blastoderm; and. Fig. 13.—Transverse Section through Embryo Chick (26 hours), a, Epiblast; b, mesoblast;c, hypoblast; d, central portion of mesoblast, which is here fused with epiblast; e, primitivegroove; /, dorsal ridge. (Klein.) this speedily differentiates into two and then into three layers, chiefly fromthe rapid proliferation of the cells of the first single layer. These layers,figure 13, are called the Epiblast, the Mesoblast, and the Hypoblast. In thefurther development of the animal it is found that from each of these layers isproduced a very definite part of the completed body. For example, fromthe cells of the epiblast are derived, among other structures, the skin and thecentral nervous system; from the mesoblast the muscles and connectivetissue of the body, and from the hypoblast the epithelium of the alimentarycanal, some of the chief glands, and so on. It is obvious that the tissues and organs so derived will exhibit in a varyingdegree the primary pro
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