CHAPTER I 



THE CELL 



Unicellular animals The cell as the starting-point in the development of the 

 higher animals (egg and sperm cells) The cells which compose the bodies 

 of those animals (tissue cells and cell tissue). 



INTRODUCTION. 



THE starting-point of all organic life and of all organic structure is the 

 cell. The simplest organisms, the lowest animals, are cells. Every 

 higher animal begins individual existence as a cell, and every higher 

 organism appears to be composed of cells which have arisen by 

 multiplication from one cell. 



The cell is an organic individual of the first order. 



In all cases except that of the lowest organisms the descendants of 

 one cell unite to form communities or states, which then rank as 

 individuals of a higher order. 



Every higher organism, every bird, every fish, and so on, is such a 

 cell community. In such a community, the closely united cells share 

 in the common work. Some undertake one function, some another, 

 for which they are specially adapted. 



Every cell consists of two essential parts : (1) the Protoplasm, and 

 (2) the Nucleus. The latter may be considered as a special differentia- 

 tion (specially developed portion) of the protoplasm. Chemically con- 

 sidered protoplasm is a carbon compound as yet not fully understood ; 

 it is related to albumen, and is, in life, a stable combination, though 

 subject to variation within very narrow limits. It is viscid and 

 capable of swelling. The nucleus lies within the protoplasm as a 

 chemically and physically differentiated part of it. It is an essential 

 portion of the cell, in whose multiplication it plays an important part. 

 According to some observers, if the nucleus is removed from a cell the 

 latter perishes. If, on the other hand, a nucleus is introduced into an 

 unnucleated piece of protoplasm, certain characteristic phenomena of 

 life otherwise absent appear in it. 



VOL. I. B 



