see.

I said just now that the Horse eventually died and became converted into
the same inorganic substances from whence all but an inappreciable
fraction of its substance demonstrably originated, so that the actual
wanderings of matter are as remarkable as the transmigrations of the
soul fabled by Indian tradition. But before death has occurred, in the
one sex or the other, and in fact in both, certain products or parts of
the organism have been set free, certain parts of the organisms of the
two sexes have come into contact with one another, and from that
conjunction, from that union which then takes place, there results the
formation of a new being. At stated times the mare, from a particular
part of the interior of her body, called the ovary, gets rid of a minute
particle of matter comparable in all essential respects with that which
we called a cell a little while since, which cell contains a kind of
nucleus in its centre, surrounded by a clear space and by a viscid mass
of protein substance (Figure 2); and though it is different in
appearance from the eggs which we are mostly acquainted with, it is
really an egg. After a time this minute particle of matter, which may
only be a small fraction of a grain in weight, undergoes a series of
changes,--wonderful, complex changes. Finally, upon its surface there is
fashioned a little elevation, which afterwards becomes divided and
marked by a groove. The lateral boundaries of the groove extend upwards
and downwards, and at length give rise to a double tube. In the upper
smaller tube the spinal marrow and brain are fashioned; in the lower,
the alimentary canal and heart; and at length two pairs of buds shoot
out at the sides of the body, which are the rudiments of the limbs. In
fact a true drawing of a section of the embryo in this state would in
all essential respects resemble that diagram of a horse reduced to its
simplest expression, which I first placed before you (Figure 1).