pellucida [a]), and contains a noncentral nucleus (the germinal
vesicle, c). Cf. Figure 1.14.)

This fact is so important that few should be unaware of its extreme
significance; yet it was quite unknown in the first quarter of the
nineteenth century. As we have seen, the human and mammal ovum was not
discovered until 1827, when Carl Ernst von Baer detected it. Up to
that time the larger vesicles, in which the real and much smaller ovum
is contained, had been wrongly regarded as ova. The important
circumstance that this mammal ovum is a simple cell, like the ovum of
other animals, could not, of course, be recognised until the cell
theory was established. This was not done, by Schleiden for the plant
and Schwann for the animal, until 1838. As we have seen, this cell
theory is of the greatest service in explaining the human frame and
its embryonic development. Hence we must say a few words about the
actual condition of the theory and the significance of the views it
has suggested.

In order properly to appreciate the cellular theory, the most
important element in our science, it is necessary to understand in the
first place that the cell is a UNIFIED ORGANISM, a self-contained
living being. When we anatomically dissect the fully-formed animal or
plant into its various organs, and then examine the finer structure of
these organs with the microscope, we are surprised to find that all
these different parts are ultimately made up of the same structural
element or unit. This common unit of structure is the cell. It does
not matter whether we thus dissect a leaf, flower, or fruit, or a
bone, muscle, gland, or bit of skin, etc.; we find in every case the
same ultimate constituent, which has been called the cell since
Schleiden's discovery. There are many opinions as to its real nature,