vary much in shape later on. Illustrations of this will be found in
the cells of the various parts of the body shown in Figures 1.3 to
1.7.

Hence the essential point in the modern idea of the cell is that it is
made up of two different active constituents--an inner and an outer
part. The smaller and inner part is the nucleus (or caryon or
cytoblastus, Figure 1.1 c and Figure 1.2 k). The outer and larger
part, which encloses the other, is the body of the cell (celleus,
cytos, or cytosoma). The soft living substance of which the two are
composed has a peculiar chemical composition, and belongs to the group
of the albuminoid plasma-substances ("formative matter"), or
protoplasm. The essential and indispensable element of the nucleus is
called nuclein (or caryoplasm); that of the cell body is called
plastin (or cytoplasm). In the most rudimentary cases both substances
seem to be quite simple and homogeneous, without any visible
structure. But, as a rule, when we examine them under a high power of
the microscope, we find a certain structure in the protoplasm. The
chief and most common form of this is the fibrous or net-like "thready
structure" (Frommann) and the frothy "honeycomb structure" (Butschli).

(FIGURE 1.2. Stem-cell of one of the echinoderms (cytula, or "first
segmentation-cell" = fertilised ovum), after Hertwig. k is the nucleus
or caryon.)

The shape or outer form of the cell is infinitely varied, in
accordance with its endless power of adapting itself to the most
diverse activities or environments. In its simplest form the cell is
globular (Figure 1.2). This normal round form is especially found in
cells of the simplest construction, and those that are developed in a