Inside it is the roundish nucleus with its nucleolus.)

When the mature bird-ovum has left the ovary and been fertilised in
the oviduct, it covers itself with various membranes which are
secreted from the wall of the oviduct. First, the large clear
albuminous layer is deposited around the yellow yelk; afterwards, the
hard external shell, with a fine inner skin. All these gradually
forming envelopes and processes are of no importance in the formation
of the embryo; they serve merely for the protection of the original
simple ovum. We sometimes find extraordinarily large eggs with strong
envelopes in the case of other animals, such as fishes of the shark
type. Here, also, the ovum is originally of the same character as it
is in the mammal; it is a perfectly simple and naked cell. But, as in
the case of the bird, a considerable quantity of nutritive yelk is
accumulated inside the original yelk as food for the developing
embryo; and various coverings are formed round the egg. The ovum of
many other animals has the same internal and external features. They
have, however, only a physiological, not a morphological, importance;
they have no direct influence on the formation of the foetus. They are
partly consumed as food by the embryo, and partly serve as protective
envelopes. Hence we may leave them out of consideration altogether
here, and restrict ourselves to material points--TO THE SUBSTANTIAL
IDENTITY OF THE ORIGINAL OVUM IN MAN AND THE REST OF THE ANIMALS
(Figure 1.13).

Now, let us for the first time make use of our biogenetic law; and
directly apply this fundamental law of evolution to the human ovum. We
reach a very simple, but very important, conclusion. FROM THE FACT
THAT THE HUMAN OVUM AND THAT OF ALL OTHER ANIMALS CONSISTS OF A SINGLE
CELL, IT FOLLOWS IMMEDIATELY, ACCORDING TO THE BIOGENETIC LAW, THAT