18 HISTOLOGY. 



next year, 18:> ( .). Schwann, encouraged by the findings of 

 Schleiden, undertook investigations on animals, and found here 

 al- > a celltdar structure. These two investigators considered the 

 cell a small vesicle containing a fluid in a definite membrane. 

 Even at this early date they thought the cell membrane and 

 nucleus to be very important, characteristic, and constant con- 

 stituents of the cell. So it was discovered that both animal and 

 vegetable organisms consist of very minute elements; and fur- 

 ther, that all these more or less complicated structures take their 

 origin from a single cell, L e., the fertilized egg. Then it was 

 shown that on the border land between the animal and vege- 

 table kingdoms unicellular creatures exist which form a 

 starting-point for both kingdoms. The original conception 

 of the cell underwent great changes in the course of the 

 following decade. 



A number of years after this, when membraneless cells had 

 been discovered, the cell membrane came to be considered as an 

 unessential part of the cell. In the ground substance of many 

 animal cells movements were observed, which were already 

 known in plant cells. These evidences of life were studied, 

 and the ground substance in animal and vegetable cells was 

 called protoplasm. 



A. THE CELL. 



What is to-day known as a cell (cellula) is a small mass of 

 protoplasm containing within it a nucleus. We must consider 

 cells as elementary units ; or, since they are the bearers of the 

 life functions, as the units of life. 



In reviewing the animal series, which is made up partly of 

 organisms consisting of only a single cell (protozoa), partly of 

 those containing a countless number of cells (metazoa), it 

 i- t< be noted that the cells of the first class subserve simul- 

 taneously different functions, while we find in the second class 

 much differentiated cells with very diverse functions. In the 

 most highly developed organisms we find these differentia- 

 tii.ns an<l this division of labor so strongly marked that one 

 kind of cell cannot take on the functions of another kind. 



