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 Ceti THEORY 
is usually dated back to the time of the botanist 
Schleiden (1838) and the zodlogist 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- 
