DEVELOPMENT. 35 



the eighteenth century. Von Mohl in 1846 recog- 

 nized in these cells a viscid, semifluid, granular sub- 

 stance which he named protoplasm. Meanwhile 

 extensive observations by other scientists demon- 

 strated the existence of cells without cell walls, all 

 of which prepared the way for Max Schultze, who in 

 1 86 1 showed conclusively the identity of protoplasm 

 in all life, establishing the protoplasm theory, another 

 great biological discovery of the eighteenth century. 

 The conception of a cell, as postulated by Schleiden 

 and Schwann, thus became modified, so that today 

 this biological term stands for a nucleated mass of 

 protoplasm which under certain conditions is capable 

 of assimilation, growth, and reproduction. In some 

 cells, as certain bacteria, no nucleus has been found; 

 however, in animal tissues as a rule, the nucleus is con- 

 stantly present and forms an essential part of the cell. 

 Whenever the nucleus is lost or destroyed, the cell dies. 

 Protoplasm. Wherever we find protoplasm we find 

 life, and wherever we find life we find protoplasm. 

 Protoplasm is not life, but all agree it is the physical 

 basis of life. Every particle of protoplasm has its 

 origin in some antecedent protoplasm, and so on, in 

 an unbroken series, all life is traced back to the earli- 

 est primitive form of protoplasm. This protoplasm 

 is a viscid, transparent, jelly-like substance. It is not 

 a substance of uniform physical and chemical proper- 

 ties, but a mixture of many organic compounds which 

 in many ways resemble the proteid bodies or the albu- 

 mins. It is insoluble in water, but absorbs water in 

 variable quantities. The protoplasm of dry seeds may 

 contain but three or four per cent, of water, while 



