most striking characteristics of the living organism.
The complicated changes connected with nuclear division, known as 'mitosis,' which underlie all cell-multiplication, and hence are of such importance in growth and development, are only realised as the result of those microscopic investigations which have been directed towards discovering a structural organisation of the cell itself.
The germ-plasm and the problems of heredity connected therewith can only be discussed in terms of cell structure with any probability of satisfactory results.
Possibly also the varied morphological characters presented by the fully developed living constituents of the tissues, developed as they have been through successive stages from cells of almost identical appearance in the blastoderm, may be more fully understood when the structure of their protoplasm is more accurately known. The so-called specificity of cells and its limitations—metaplastic interchanges—so important in the study of tumour formation, is an aspect of this same question.
From the earliest recorded times there has prevailed an idea which ascribed to matter an ultimate composition of indivisible indestruct-
