Page:Popular Science Monthly Volume 67.djvu/121

Rh and relatively simple. Given a leaf-like rudiment, with the layers held out by the yolk, as is the case in the hen's egg, and it was no easy matter to conceive of how they are transformed into the nervous system, the body wall, the alimentary canal and other parts, but, Von Baer saw deeply and clearly that the fundamental anatomical features of the body are assumed by the leaf-like rudiments being rolled into tubes. Fig. 8 shows four sketches taken from the plates illustrating Von Baer's work. At A is shown a stage in the formation of the embryonic

envelope, or amnion, which surrounds the embryos of all animals above the class of amphibia. At B, another figure of an ideal section, shows that long before the day of microtomes, Von Baer made use of sections to represent the relationships of his four germ-layers. At C and D is represented, diagramatically, the way in which these layers are rolled into tubes. He showed that the central nervous system arose in the form of a tube, from the outer layer, the body-wall in the form of a tube, composed of skin and muscle layers, and the alimentary tube from mucous and vascular layers.

The generalization that embryos in development tend to recapitulate their ancestral history is frequently attributed to Von Baer, but the qualified way in which he suggests something of the sort will not justify one in attaching this conclusion to his work.

Von Baer was the first to make embryology truly comparative, and to point out its great value in anatomy and zoology. By embryological