VON BAER AND RISE OF EMBRYOLOGY. 



119 



Fig. 10. A. Kowalevsky 1840-1901. 



Kowalevsky's work was to break down the sharply limited line, sup- 

 posed 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 gastraea theory. 



Beginning of the Idea of Germinal Continuity. — The conception 

 that there is unbroken continuity of 

 germinal substance between all liv- 

 ing 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 concep- 

 tion, 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 ' pub- 

 lished in 1858. It was not, how- 

 ever, 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 fol- 

 lowed 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 sur- 

 vivals, 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 Balfonr proved to be one of 

 great importance on account of the gonoral advances in knowledge of 



