INTRODUCTION 3 



with fluid were slightly later described by Malpighi. 

 In 1833 Robert Brown discovered nuclei in certain 

 plant cells. What is known now as the Cell Theory 

 is usually dated back to the time of the botanist 

 Schleiden (1838) and the zoologist Schwann (1839), 

 whose investigations of the cellular phenomena in 

 animals and plants added greatly to the knowledge 

 of these units of structure. At this time the cell- 

 wall was considered the important part of the cell, 

 but continued research proved this idea to be erro- 

 neous. Schleiden called the substance within the cells 

 plant slime. Later (1846) von Mohl gave the term 

 protoplasm to the same substance. The substance 

 within the animal cell was named sarcode by Du- 

 jardin. The similarities between the protoplasm of 

 plants and the sarcode of animals were noted by 

 Cohn, and animal cells without cell-walls were 

 observed by Kolliker (1845). It was not, however, 

 until 1861 that Max Schultze finally established the 

 fact that plant protoplasm and animal sarcode are 

 essentially alike, and defined the cell as a mass of 

 protoplasm containing a nucleus. Schultze's re- 

 searches serve as the starting point for modern 

 studies of cellular phenomena, but the definition 

 furnished by him must be modified slightly, since we 

 now know that many cells exist without definite 

 nuclei. These cells, however, are provided with 

 nuclear material scattered throughout the cell body 

 (the so-called distributed nucleus). Our definition 

 must be changed to read, a cell is a mass of proto- 

 plasm containing nuclear material. Changes like- 



