98 The American Naturalist. [February, 
words, structural differentiation has begotten or accompanied 
chemical and functional differentiation. All this seems quite 
plausible, indeed, quite natural, but how shall we explain all 
the varied functions possessed by the unicellular organism 
unless we accept the idea of a possible chemical differentiation 
of the cell protoplasm inside the cell wall. Digestion, assimi- 
lation, excretion and reproduction are functions possessed 
alike by the unicellular organism and its higher neighbor the 
multicellular organism. In the latter, we recognize distinct 
groups of specially characterized cells for each phase and form 
of functional activity, each group as in a gland or tissue hav- 
ing a different chemical structure with its own peculiar line of 
chemical activity and its own particular katabolic products. 
In the unicellular organism, on the other hand, a differentia- 
tion of protoplasmic particles is the only plausible explanation 
of the diverse functions of the living cell. 
This being true we can no longer look on the cell as the 
ultimate unit of structure, certainly not from the chemical 
standpoint. The cell may be considered rather as a complex 
molecule, or series of molecules, built up of many morphologi- 
cal atoms or rather groups of atoms. Thus, the cytoplasm, for 
example, may be looked upon as a multitude or mass of living 
units of structure, as the plasomen of Wiesner. Call them 
what you will—plasomen, idiosomes, gemmules, plastidules, 
idioblasts or physiological units—these particles have the 
power of dividing, and, indeed, of growth and assimilation. 
Moreover, it is possible that this power of growth and repro- 
duction may be independent, in part at least, of the cell nucleus 
and the constituent karyoplasm. Furthermore, the nucleus 
too may perhaps be considered as composed of auto-divisible 
organic individuals, these hypothetical particles, both of the 
cytoplasm and of the karyoplasm, being considered as the 
living atoms of the molecule, the last divisible living bodies 
of the cell. ; 
For fifty years or more, the cell theory of structure and 
development as outlined by Schleiden and Schwann has bee? 
the nucleus for nearly all phases of biological work, and 
although our knowledge of the cell has advanced greatly 
