THE RISE OF EMBRYOLOGY 



225 



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 in\'olved 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 \'irchow's Cellular Pa- 

 thology, published in 1S58. It was not, however, until the 



Fig. 68. — A. Kowalevsky, 1840-1901. 



period of Balfour, and through the work of Fol, \^an Beneden 

 (chromosomes, 1883), Bovcri, Hertwig, and others, that the 

 great importance of this conception began to be appreciated, 

 and came to be woven into the fundamental ideas of de- 

 velopment. 



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 



Darvv'in, in 1859, brought a new feature into its discussion 



15 



