Page:Popular Science Monthly Volume 67.djvu/120

114 should not be overlooked. In the second place, he established the germ-layer theory, and, in the third, he made embryology comparative.

In reference to the germ-layer theory, it should be recalled that Wolff had distinctly foreshadowed the idea, by showing that the material out of which the embryo is constructed is, in an early stage of development, arranged in the form of leaf-like layers. He showed specifically that the alimentary canal is produced by one of these sheet-like expansions folding and rolling together.

Pander, by observations on the chick (1817), had extended the knowledge of these layers and elaborated the conception of Wolff. He recognized the presence of three primary layers, an outer, a middle and an inner, out of which the tissues of the body are formed.

But, it remained for Von Baer, by extending his observations into all the principal groups of animals, to raise this conception to the rank of a general law of development. He was able to show that in all animals except the very lowest, there arises in the course of development leaf-like layers, which become converted into the 'fundamental organs' of the body.

Now, these elementary layers are not definitive tissues of the body, but are embryonic, and therefore, may appropriately be designated 'germ-layers.' The conception that these germ-layers are essentially similar in origin and fate, in all animals, was a fuller and later development of the germ-layer theory, which dominated embryological study until a recent date.

Von Baer recognized four such layers: the outer and inner ones being formed first, and, subsequently budding off a middle layer composed of two sheets. A little later (1845) Remak recognized the double middle layer of Von Baer as a unit, and thus arrived at the fundamental conception of three layers—the ecto-, endo-and mesoderm—which has so long held sway. For a long time after Von Baer, the aim of embryologists was to trace the history of these germ-layers—and so in a wider and much qualified sense it is to-day.

It will ever stand to his credit, as a great achievement, that Von Baer was able to make a very complicated feature of development clear