THE RISE OF EMBRYOLOGY 223 



recognized as modified cells. This position was reached, 

 for the egg, about 1861, when Gegcnbaur showed that the 

 eggs of all vertebrate animals, regardless of size and con- 

 dition, 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 arrange- 

 ment 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 arc, in reality, similar cells per- 

 forming a similar function. Finally, from combinations of 

 tissues, the organs are formed. 



But the egg, before entering on the process of develop- 

 ment, 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, men- 

 tioned 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 



