Healthcare and Medicine Reference
Fig. 1.33 When the ovum is fertilized, its membrane and the
gummy zona pellucida surround the same space (A). With the first
cell division, the two-celled organism is held in place by the
metamembrane of the zona (B). The zona persists as the
organismic limit right up through the blastocyst stage.
around the zygote forms the first metamembrane for the
organism. This is the first of the connective tissue prod-
ucts to do so, later to be joined by the fibrillar elements
of reticulin and collagen. But this exudate is the initial
organismic environment, and the original organismic
With the first division, a small amount of cytoplasm
escapes the two daughter cells, forming a thin film of
fluid surrounding the two cells, and between the cells
and the zona pellucida (Fig. 1.33B). 8 0 This is the first hint
of the fluid matrix, the lymphatic or interstitial fluid that
will be the main means of exchange among the com-
munity of cells within the organism.
We can also note that while the single cell is organized
around a point, the two-celled organism is organized
around a line drawn between the two centers of the cells.
The early zygote will alternate between these two -
organization around a point, then organization around a
line. Further, the two-celled organism resembles two bal-
loons (two pressurized systems) pushed together, so that
their border is a double-layered diaphragm, another
popular shape throughout embryogenesis.
The cells continue to divide, creating a 50-60-cell
morula (bunch of berries) within the confines of the
zona (see Fig. 1.32). After five days, the zona has thinned
and disappeared, and the morula expands into a blasto-
sphere (Fig. 1.34A), an open sphere of cells (which thus
echoes in shape the original sphere of the ovum).
In the 2nd week of development, this blastosphere
invaginates upon itself during gastrulation (Fig. 1.34B).
Gastrulation is a fascinating process where certain cells
in one 'corner' of the sphere send out pseudopods which
attach to other cells, and then, by reeling in the exten-
sions, create first a dimple, then a crater, and finally a
tunnel that creates an inner and an outer layer of cells
(Fig. 1.34C). 8 1 This is the basic double-bag shape, a sock
turned halfway inside-out or a two-layered cup. Notice
that this ancient tunicate-like shape creates three poten-
Fig. 1.34 The first definitive autonomous motion of the embryo is
to fold the blastosphere in upon itself to form a double bag, which
connects the epiblast and hypoblast into the bilaminar membrane.
This motion forms the first double bag.
If the 'mouth' of the structure is open, then there is
no difference between space 1 and space 3, but if the
sphincter of the mouth is closed, they are three distinct
areas separated by the two bags.
This inversion results in the double bags of the
amnion and yolk sac, with the familiar trilaminar disc
of ectoderm, mesoderm, and endoderm sandwiched
between (Fig. 1.35 - note the similarity to the two-celled
shape in Fig. 1.33B). The ectoderm, in contact with the
amniotic sac and fluid, will form the nervous system
and skin (and is thus associated with the 'neural net' as
described above). The endoderm, in contact with the
yolk sac, will form the linings of all our circulatory
tubing, as well as the organs of the alimentary canal,
along with the glands (and is the primary source of the
fluid vascular net). The mesoderm in between the two
will form all the muscles and connective tissues (and is
thus the precursor of the fibrous net), as well as the
blood, lymph, kidneys, most of the genital organs, and
the adrenal cortex glands.
The formation of the fascial net
If we may digress from double-bagging for a moment
to follow the development of the fibrous net within the
embryo: this initial cellular specialization within the
embryo, which occurs at about two weeks' develop-
1. the space within the inner bag;
2. the space between the inner and the outer bag;
3. the environment beyond the outer bag.