spot. Even when we use the most powerful microscope with its highest
power, we can detect no material difference between the ova of man,
the ape, the dog, and so on. I do not mean to say that there are no
differences between the ova of these different mammals. On the
contrary, we are bound to assume that there are such, at least as
regards chemical composition. Even the ova of different men must
differ from each other; otherwise we should not have a different
individual from each ovum. It is true that our crude and imperfect
apparatus cannot detect these subtle individual differences, which are
probably in the molecular structure. However, such a striking
resemblance of their ova in form, so great as to seem to be a complete
similarity, is a strong proof of the common parentage of man and the
other mammals. From the common germ-form we infer a common stem-form.
On the other hand, there are striking peculiarities by which we can
easily distinguish the fertilised ovum of the mammal from the
fertilised ovum of the birds, amphibia, fishes, and other vertebrates
(see the close of Chapter 2.29).

(FIGURE 1.13. Ova of various animals, executing amoeboid movements,
highly magnified. All the ova are naked cells of varying shape. In the
dark fine-grained protoplasm (yelk) is a large vesicular nucleus (the
germinal vesicle), and in this is seen a nuclear body (the germinal
spot), in which again we often see a germinal point. Figures A1 to A4
represent the ovum of a sponge (Leuculmis echinus) in four successive
movements. B1 to B8 are the ovum of a parasitic crab (Chondracanthus
cornutus), in eight successive movements. (From Edward von Beneden.)
C1 to C5 show the ovum of the cat in various stages of movement (from
Pfluger); Figure P the ovum of a trout; E the ovum of a chicken; F a
human ovum.)