analyzed into their units; now, for the first time, was comprehended the nature of the germ-layers of Von Baer.
Among the first questions to emerge in the light of the new researches were: What is the origin of the cells in the organs, the tissues and the germ-layers? The road to the investigation of these questions was already opened, and it was followed, step by step, until the egg and sperm came to be recognized as modified cells. This position was reached, for the egg, about 1861, when Gegenbaur showed that the eggs of all vertebrated animals, regardless of size and condition, are in reality single cells. The sperm was put in the same category about 1865.
The rest was relatively easy—the egg, a single cell—by successive divisions produces many cells, and the arrangement of these into primary embryonic layers brings us to the starting point of Wolff and Von Baer. The cells, continuing to multiply by division, not only increase in number, but also undergo changes through division! of physiological labor, whereby certain groups are set apart to perform a particular part of the work of the body. In this way arise the various tissues of the body—which are, in reality, similar cells performing a similar function. Finally, from combinations of tissues the organs are formed.
But the egg, before entering on the process of development, must be stimulated by the union of the sperm with the nucleus of the egg, and, thus, the starting point of every animal and plant, above the lowest group, proves to be a single cell with protoplasm derived from two parents. While questions regarding the origin of cells in the body were being answered, the foundation for the embryological study of heredity was also laid.
Advances were now more rapid and more sure, flashes of morphological insight began to illuminate the way, and the facts of isolated observations began to fit into a harmonized whole.
Apart from the general advances of this period, mentioned in other connections, the work of a few individuals requires notice.
Bathke and Remak were engaged with the broader aspects of embryology as well as with special investigations. To Rathke is owing great advances in the knowledge of the development of insects and other invertebrates, and Remak is notable for similar work with the vertebrates. As already mentioned, he was the first to recognize the middle layer as a unit—through which the three germ-layers of later embryologists emerged into the literature.
Koelliker, the veteran embryologist, still living in Würzburg, carried on investigations on the segmentation of the egg. Besides work on the invertebrates, later, he followed with care the development of the chick and the rabbit—he encompassed the whole field of embryology—and published, in 1861 and later, in 1876, a general treatise on
