16 THE MAIN CURRENTS OF ZOOLOGY 



the protoplasm idea, Dujardin (Fig. i) and Max 

 Schultze (Fig. 2) occupy the foremost position, a 

 great many other investigators, as the botanists 

 De Bary, Nageli, Strassburger and others contrib- 

 uted to this end. 



The Cell-Theory. The cell-theory had a parallel 

 development with the protoplasm idea and, ulti- 

 mately, they fused as one. The cell-theory was an- 

 nounced in 1838-1839, a few years after Dujardin's 

 discovery of protoplasm. 



The microscopic examination of a thin section of a 

 plant stem, a similar section of hardened liver, 

 scrapings from the inside of the human cheek, the 

 skin of a frog, the skin, or epidermis, of a plant, all 

 reveal similar units of organic architecture. Further 

 studies show that brain tissue, bone and cartilage, in 

 fact, all organic tissues are constructed by the union 

 of microscopic elements nicely fitted in together. 

 This is the basis of the cell-theory, but it is a long 

 step from mere observation of these elements to the 

 generalization that all animals and plants are com- 

 posed of a union of similar cells. The latter concep- 

 tion in its full sweep unites all living creations on the 

 broad plane of similarity of structure and, as we shall 

 soon see, of similarity of origin since all organisms, 



