THE RISE OF EMBRYOLOGY 223 
recognized as modified cells. This position was reached, 
for the egg, about 1861, when Gegenbaur 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 are, 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 
