222 BIOLOGY AND ITS MAKERS 
of all the principal organs from the germ-layers, instinctively 
getting at the truth as only a great genius could have done.” 
After his masterly work, the science of embryology could 
never return to its former level; he had given it a new direc- 
tion, and through his influence a period of great activity was 
introduced. 
THE PERIOD FROM VON BAER TO BALFOUR 
In the period between Von Baer and Balfour there were 
great general advances in the knowledge of organic structure 
that brought the whole process of development into a new 
light. 
Among the most important advances are to be enumerated 
the announcement of the cell-theory, the discovery of proto- 
plasm, the beginning of the recognition of germinal continuity, 
and the establishment of the doctrine of organic evolution. 
The Cell-Theory.—The generalization that the tissues of 
all animals and plants are structurally composed of similar 
units, called cells, was given to the world through the com- 
bined labors of Schleiden and Schwann. ‘The history of this 
doctrine, together with an account of its being remodeled 
into the protoplasm doctrine, is given in Chapter XII. 
The broad-reaching effects of the cell-theory may be easily 
imagined, since it united all animals on the broad place of 
likeness in microscopic structure. Now for the first time 
the tissues of the body were analyzed into their units; now 
for the first time was comprehended the nature of the germ- 
layers of Von Baer. 
Among the first questions to emerge in the light of the new 
researches were concerning the origin of cells in the organs, 
the tissues, and the germ-layers. The road to the investiga- 
tion of these questions was already opened, and it was fol- 
lowed, step by step, until the egg and the sperm came to be 
