out of the flat spinal plate a hollow nerve-tube, from the skin-plate
a skin-tube, and so on.

Among the many great services which Baer rendered to embryology,
especially vertebrate embryology, we must not forget his discovery of
the human ovum. Earlier scientists had, as a rule, of course, assumed
that man developed out of an egg, like the other animals. In fact, the
preformation theory held that the germs of the whole of humanity were
stored already in Eve's ova. But the real ovum escaped detection until
the year 1827. This ovum is extremely small, being a tiny round
vesicle about the 1/120 of an inch in diameter; it can be seen under
very favourable circumstances with the naked eye as a tiny particle,
but is otherwise quite invisible. This particle is formed in the ovary
inside a much larger globule, which takes the name of the Graafian
follicle, from its discoverer, Graaf, and had previously been regarded
as the true ovum. However, in 1827 Baer proved that it was not the
real ovum, which is much smaller, and is contained within the
follicle. (Compare the end of Chapter 2.29.)

Baer was also the first to observe what is known as the segmentation
sphere of the vertebrate; that is to say, the round vesicle which
first develops out of the impregnated ovum, and the thin wall of which
is made up of a single layer of regular, polygonal (many-cornered)
cells (see the illustration in Chapter 1.12). Another discovery of his
that was of great importance in constructing the vertebrate stem and
the characteristic organisation of this extensive group (to which man
belongs) was the detection of the axial rod, or the chorda dorsalis.
There is a long, round, cylindrical rod of cartilage which runs down
the longer axis of the vertebrate embryo; it appears at an early
stage, and is the first sketch of the spinal column, the solid