250 BIOLOGY AND ITS MAKERS 



One comes from the reading of these two contributions 

 to science with the feeling that it is really Schwann's cell- 

 theory, and that Schleiden helped by lighting the way that 

 his fellow-worker so successfully trod. 



Modification of the Cell-Theory. — The form in which the 

 cell-theory was given to the world by Schleiden and Schwann 

 was very imperfect, and, as already pointed out, it contained 

 fundamental errors. The founders of the theory attached 

 too much importance to the cell -wall, and they described the 

 cell as a hollow cavity bounded by walls that were formed 

 around a nucleus. They were wrong as to the mode of the 

 development of the cell, and as to its nature. Nevertheless, 

 the great truth that all parts of animals and plants are built 

 of similar units or structures was well substantiated. This 

 remained a permanent part of the theory, but all ideas re- 

 garding the nature of the units were profoundly altered. 



In order to perceive the line along which the chief modifi- 

 cations were made we must take account of another scientific 

 advance of about the same period. This was the discovery 

 of protoplasm, an achievement which takes rank with the 

 advances of greatest importance in biology, and has proved 

 to be one of the great events of the nineteenth century. 



The Discovery of Protoplasm and its Effect on the Cell- 

 Theory. — In 1835, before the announcement of the cell- 

 theory, living matter had been observed by Dujardin. In 

 lower animal forms he noticed a semifluid, jelly-like sub- 

 stance, which he designated sarcode, and which he described 

 as being endowed with all the qualities of life. The same 

 semifluid substance had previously caught the attention of 

 some observers, but no one had as yet announced it as the 

 actual living part of organisms. Schleiden had seen it and 

 called it gum. Dujardin was far from appreciating the full 

 importance of his discovery, and for a long time his descrip- 

 tion of sarcode remained separate; but in 1846 Hugo von 



