132 



CELLS OF THE VERTEBRATE BODY 



studies (Fig. 71). Robert Hooke, one of the early English micros- 

 copists, discovered that cork was composed of small spaces, sur- 

 rounded by firm walls, and in 1665 he gave the name cell to these 

 compartments (Fig. 72). The development of microscopic instru- 

 ments was very slow, and it was not until 1833 that Robert Brown 

 observed, in certain plant structures, that each cell contained a 

 small body, which he called the nucleus. In 1838, Schleiden, a 



German botanist, 

 proposed the inter- 

 pretation that cells 

 were the units of 

 structure in plants, 

 and Schwann in 

 1839 extended this 

 conception to the 

 -A. Diagram of cork to show cell walls of gtructure of ani- 

 plant cells. S. Squamous epithelium to show nucleus , rFio- 73") 



and protoplasm in the animal cell. There are no . ,' n 1 



cell walls. This was the first 



cw, cell wall; n, nucleus; /(, protoplasm. lOmiUlatlOn OI tue 



Cell Theory. 



The founders of this theory, and other scientists of that time, 

 believed that the walls that surround plant cells were the essential 

 part of these units. The contents of cells had been observed, but 

 were regarded as unimportant or as waste products. Purkinje, 

 in 1840, and von Mohl, in 1846, gave the name protoplasm to Uie 

 cell contents. Through a series of researches, it became apparent 

 that protoplasm was the essential part of cells, since walls were 

 found only in plant cells. Likewise, the presence of a nucleus 

 was discovered in almost all types of cells, and a cell came to be 

 described as a mass of protoplasm containing a nucleus. 



As knowledge became more complete it was ascertained that in 

 certain parts of the animal much material which is not in the form 

 of cells lies between them. This material, examples of which will 

 be given later, was shown to be produced by cells, and the Cell 

 Theory was modified by saying that organisms are composed 

 of structural units, called cells, and of cell products. Further 

 study of animals with reference to their activities has revealed 

 the fact that all physiological processes must be understood in 

 terms of the functions of cells. That is, the cell is the unit of 

 function. The Cell Theory has, therefore, been extended and 



