150 BOARD OF AGRICULTURE. 



substance within an enveloping membrane. Within a cell we 

 usually expect to find a nucleus, or, possibly, within the 

 nucleus another cell, which we call a nucleolus. These inner 

 cells are almost invariably of a round or oval form, offer greater 

 resistance to the action of chemical agents than do the exter- 

 nal parts, and are those parts which are the most constantly 

 found unchanged. The nucleus seems less connected with 

 the function and specific office of the cell, according to 

 Virchow, than with its maintenance and multiplication as a 

 living part. 



For the existence of the cellular element, such as we are to 

 consider, two things are requisite, — the membrane and the 

 nucleus. The contents change according to position and 

 function. With these two forms, — the membrane and the 

 nucleus, — we are enabled to examine critically the basis of 

 some of the phenomena attending life. 



In the embryouic state we can readily detect the time when 

 the whole structure is composed of cells, and as we pass 

 onward towards birth, we see these cells changing their form 

 and function, becoming differentiated as it were, in an increas- 

 ing ratio with the age. The cells multiply, change their form 

 and their function, which necessarily involves their contents, 

 until in the grown individual it is difficult to trace the connec- 

 tion between these elements in the various parts. The cells 

 change, but while the activity of the cell remains, the nucleus 

 can usually be detected. The muscle-cells become elongated 

 and become filled with contractile matter, and capable of 

 transmitting force ; the nucleus remains attached somewhere 

 to this cell and is unchanged. So with the nerve-cells ; the 

 contents differ from the muscle-cells, and it is but the nucleus 

 which remains to indicate the kinship. We also find changes 

 going on in the shapes of cells by outgrowths, by division, by 

 absorption, and even by secretion and growth. These cells 

 containing nuclei are, in fact, individual units of a living 

 organism, and themselves containing life and undergoing vital 

 processes, go to make up the concrete life we recognize in the 

 formed animal. 



As the processes which these vital unities pass through are 

 all allied, Ave can consider some of the laws of reproduction, as 

 derived from the study of the simple or ideal cell, leaving our 



