The first week:
a floating and timeless existence
The first cell
Once the egg and the sperm cell have fused, the nuclei fuse and the
chromosomes form pairs. The zona pellucida closes, so that no more
sperm cells can connect with the egg. The zygote (= fertilized egg) has
come into being. For a few days there is no exchange of
nutrients and other substances with the environment.
The unicellular zygote exists for about one day. Then the
cell divides itself into two equally big cells. Then they divide
again every 16 to 20 hours. First there are four, then eight, then
sixteen cells. On the fifth day there are about 32 cells and then about
64, the divisions are no longer synchronized. The stage of 16 to 64
cells is called a morula (= mulberry, the zygote resembles a mulberry
or a blackberry).
On the sixth day, most cells migrate to the periphery
(called the trophoblast (= nutritive tissue)) and a clump of cells
stays inside (called the embryoblast (= embryonic tissue)). The
trophoblast cells move closer to one another which makes the tissue
firmer. The divisions continue. On the sixth day, there are
approximately 120 cells, of which some 10 form the inner embryonic cell
mass and about 110 the outer trophoblast, the ratio is about 1 : 10.
All divisions occur within the shell of the zona pellucida. The
embryo does not grow. The cells become smaller after each division and
contain relatively less cytoplasm and more nuclear material.
As this happens, the embryo floats passively on the stream of fluid in
the oviduct as it is transported to the uterus.
On the sixth day when the zygote is in the uterus, the zona pellucida
breaks through. This happens
because in the trophoblast on the side of the blastocoel enzymes are
formed that make the zona pellucida softer. Simultaneously the embryo
expands to 2 times its volume, then contracts to 50% and
expands again to 3
times its volume. Then the zona pellucida breaks open and the
embryo bulges out of its shell. This "first
birth" is called hatching and can be compared to the hatching
of a chicken (Fig. 9). The embryo is now out of its rigid zona
pellucida and can start to grow.
The release of the embryo from its shell is an active
process of the embryo itself. The softening of the zona pellucida,
together with the expansion, cause a break which enables the embryo to
Length of time
Remarkably, the development of the zygote up to the blastula stage is
or less the same in all mammals; it lasts approximately one week. This
is not only the case in humans, with their embryonic development of 9
months, but also in mice with a development time till birth of 21 days
and the elephant (21 months): the first phase up to the blastula of all
of them lasts 6 days. For the rest of
the development of mice there are only about 15 days left.
Roe deer have an extended gestation time. The rut takes place in
July/August. The zygote develops into a blastula and then rests for
approximately 150 days, this is a so-called embryopause. No
earlier than in December the development goes on with an implantation.
After a further development of 144 days the two fawns are born in
May/June. The roe deer is the only ungulate
in which this process takes place, but an extended gestation
period is also seen with mustelids (including the badger), bears and
kangaroos. The embryopause always takes place at the end of the first
week in the blastula stage of the embryo, before implantation.
The first day after conception, a human is (or better: we are) a large
round cell. That's all we are, there is no more, but no less either. It
seems a perfect life: the one-cell human organism is an entity and the
one cell includes everything. The zygote is round and floats. We can
easily see that the entity is first and the parts come later. A human
is not built from cells of organs, a human divided into cells
and into organs.
After the first day there are divisions and after 6 days the blastula
is formed: a sphere with an interior space. The embryo has no
direction: left and right, up and down are not yet concepts to describe
it. Directions come later.
Immediately after the conception the zona pellucida closes and
there is no interaction with the environment. After 6 days of cell
divisions the cells become very small. There is now
more nuclear material and less cytoplasm than on the first day,
which decreases the inner activity of the cells. The embryo has become
more structured compared to the zygote. It has now more the structured
quality of the sperm,
the inner mobile egg quality from the first day has diminished.
this is that the morula is easier to freeze than the egg (the sperm is
also easy to freeze).
process of dividing, at the same time as the cells are shrinking,
cannot continue because the cells would become too small and the embryo
would become rigid and die. There must be a reversal. This reversal is
brought about by the blastula expanding and contracting
and finally bulging out of the zona pellucida. Then it can
absorb nutrients and start to grow and the cells can grow too. The
embryo will make contact with the uterine wall and its development can
continue. Estimates are that this process occurs with only 50 to 70% of
In mammals the development of the embryo until the implantation takes
6 days. In
the rest of the time of the embryonic development the differences are
big: from 15
days to 21 months. The duration of the development of all mammals to
the blastula has
its own time and is not species-specific. It is not part of the life of
the animal, it has its own life. It is as if (species related) time has
not yet arrived.
Summary of the
- The embryo is a sphere, there are no directions.
- The development is set
within the seclusion of the zona pellucida.
- Entity or
unity comes first, multiplicity comes later.
- The duration of the development
in the first week is fixed and equal for all species of mammals.
the mineral are, that it falls apart into the same elements;
there is no
communication with the environment and there is no time. The first week
of the embryo can therefore be called the "mineral human
The divisions of day 1 - day 6
1st row: The
of two cells after 24 hours, these divisions are regular: 4 and 8
cells, around the zygote is the zona pellucida;
Second row: after about 96 hours (5 days) there are 32 cells,
the morula has arisen; early blastula with embryoblast and trophoblast
on day 6; late blastula on day 7 without
The last two drawings are cross-sections. The colour has no meaning.
the time is variable, it can be one day earlier or later
See also: www.embryology.ch
Figure 9. The zona
pellucida breaks open on the side of the blastocoel
of the blastula and bulges outward
Figure 10. The
uterus, fallopian tube and ovary.
In the ovary the
stages of the maturation of the egg cell are shown. In the
fallopian tubes the first cell
divisions have been drawn as well as the places where they
approximately take place.
Figure 11. The
mineral or a picture for the "mineral human" (from van der Wal
The mineral is
split into equal components, there no communication with the
environment and no