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 ccll-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 thcory, 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 
