Kowalevsky's work 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 Idea of Germinal Continuity.—The conception that there is unbroken continuity of germinal substance between all Fig. 10. A. Kowalevsky 1840-1901. living organisms, and that the egg and 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 the fact began to be appreciated, and the conception began to be woven into the fundamental ideas of development.

Influence of the Doctrine of Organic Evolution.—This doctrine, although founded in its modern sense by Lamarck, in the early part of the nineteenth century, lay dormant until Darwin, in 1859, brought a new feature into its discussion, by emphasizing the factor of natural selection. The general acceptance of the doctrine, which followed after fierce opposition, had, of course, a profound influence on embryology. The latter science is so intimately concerned with the genealogy of animals and plants, that the newly accepted doctrine, as affording an explanation of this genealogy, was what was most needed. The development of organisms was now seen in the light of ancestral history; rudimentary organs began to have meaning as hereditary survivals, and the whole process of development assumed a different aspect. This doctrine supplied a new impulse to the interpretation of nature at large, and of the embryological record in particular. The meaning of the embryological record was so greatly emphasized in the period of Balfour, that it will be commented upon under the next division of our subject.

The period between Von Baer and Balfour proved to be one of great importance on account of the general advances in knowledge of