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.
Rathke and Remak were engaged with the broader aspects of embryology, as well as with special investigations. From Rathke's researches came 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 of the subject.
Koelliker, 1817-1905, the veteran embryologist, for so many years a professor in the University of 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 again in 1876, a general treatise on vertebrate embryology, of high merit. The portrait of this distinguished man is shown in Chapter VIII, where also his services as a histologist are recorded.
Huxley took a great step toward unifying the idea of germ-layers throughout the animal kingdom, when he maintained, in 1849, that the two cell-layers in animals like the hydra and oceanic hydrozoa correspond to the ectoderm and endoderm of higher animals.
Kowalevsky (Fig. 68) made interesting discoveries of a general bearing. In 1866 he showed the practical identity, in the early stages of development, between one of the lowest vertebrates (amphioxus) and a tunicate. The latter up to that time had been considered an invertebrate, and the effect of Kowalevsky's observations was to break down the sharply limited line supposed to exist between the invertebrates and the vertebrates. This was of great influence in subsequent work. Kowalevsky also founded the generalization that all animals in development pass through a gastrula stage—a doctrine associated, since 1874, with the name of Haeckel under the title of the gastræa theory.
Beginning of the Doctrine of Germinal Continuity.—The conception that there is unbroken continuity of germinal substance between all living organisms, and that the egg and the sperm are endowed with an inherited organization of great complexity, has become the basis for all current theories of heredity and development. So much is involved in this conception that, in the present decade, it has been designated (Whitman) "the central fact of modern biology." The first clear expression of it is found in Virchow's Cellular Pathology, published in 1858. It was not, however, until the period of Balfour, and through the work of Fol, Van Beneden (chromosomes, 1883), Boveri, Hertwig, and others, that the great importance of this conception began to be appreciated, and came to be woven into the fundamental ideas of development.
Fig. 68.—A. Kowalevsky, 1840-1901.