We have now, by our study of the ovum and the comparison of it with
the amoeba, provided a perfectly sound and most valuable foundation
for both the embryology and the evolution of man. We have learned that
the human ovum is a simple cell, that this ovum is not materially
different from that of other mammals, and that we may infer from it
the existence of a primitive unicellular ancestral form, with a
substantial resemblance to the amoeba.

The statement that the earliest progenitors of the human race were
simple cells of this kind, and led an independent unicellular life
like the amoeba, has not only been ridiculed as the dream of a natural
philosopher, but also been violently censured in theological journals
as "shameful and immoral." But, as I observed in my essay On the
Origin and Ancestral Tree of the Human Race in 1870, this offended
piety must equally protest against the "shameful and immoral" fact
that each human individual is developed from a simple ovum, and that
this human ovum is indistinguishable from those of the other mammals,
and in its earliest stage is like a naked amoeba. We can show this to
be a fact any day with the microscope, and it is little use to close
one's eyes to "immoral" facts of this kind. It is as indisputable as
the momentous conclusions we draw from it and as the vertebrate
character of man (see Chapter 1.11).

(FIGURE 1.19. Blood-cells that eat, or phagocytes, from a naked
sea-snail (Thetis), greatly magnified. I was the first to observe in
the blood-cells of this snail the important fact that "the blood-cells
of the invertebrates are unprotected pieces of plasm, and take in
food, by means of their peculiar movements, like the amoebae." I had
(in Naples, on May 10th, 1859) injected into the blood-vessels of one
of these snails an infusion of water and ground indigo, and was