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 \^on Baer to Balfour 



In the period between \'on 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 e\'olution. 



The Cell-Theory. — The generalization that the tissues of 

 all animals and ])lants 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 jjrotoplasm 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 w^as already opened, and it was fol- 

 lowed, step by step, until the egg and the sperm came to be 



