Animal Development - Animal Embryos
When the animal egg is fertilised by the spermatozoan the egg first erects a fertilisation membrane to
prevent additional spermatozoa from fertilising it. The fertilised egg, now a
zygote, soon undergoes a
series of cell divisions to produce a multicellular embryo - a process called
cleavage. Each division results
in two daughter cells. During its development the egg develops polar symmetry - it acquires a top and a
bottom. The nucleus of the egg (which is a single cell) sits closer to one of the poles, called the
animal (or
cytoplasmic) pole
and the yolk tends to accumulate toward the opposite or vegetal pole. We usually draw
the animal pole topmost. The first cell division occurs in a vertical plane, from animal to vegetal poles,
splitting the cell into two.

There are two main types of division pattern that follow on from the two-cell stage. The
radial cleavage
pattern is the simplest to understand and is shown above. This occurs in vertebrates (e.g. mammals like
the human) and echinoderms (e.g. starfish) and hemichordates (e.g. acorn worm) - animals known as
deuterostomes. The second division occurs also in a vertical plane, but one which is at right angles to the
first division plane and results in 4 cells, more-or-less in the same plane. The third division is a horizontal
one and gives rise to the 8-cell stage which consists of two layers of 4 cells each, with the cells of the top
layer directly above the cells of the bottom layer. This division is often approximately equal, producing
cells of equal size, but in other cases the top layer of cells may be slightly smaller as the larger vegetal
cells may contain more yolk.

The second principle type of cleavage is spiral cleavage and occurs in the so-called protostomes, animals
such as annelid (segmented) worms, e.g. polychaetes, arthropods and molluscs and is shown below:
In spiral cleavage the divisions, especially from the third upwards, are very unequal. The third division
creates 4 larger cells called macromeres and 4 smaller cells called micromeres. The macromeres form a
layer of 4 cells at the bottom or vegetal pole and the micromeres are located at the animal pole. The
macromeres are larger principally because they receive more of the yolk (which was originally mostly
vegetal in position in the egg). Even at the 4-cell stage, one of the cells may contain most of the yolk
and be distinctly larger than the other three. The defining feature of spiral cleavage, however, is the fact
that each successive layer of cells rotates so as to sit in-between the two adjacent cells of the lower
layer, in alternating manner. In polychaetes, the first layer of 4 micromeres are rotated approximately 45
degrees clockwise as viewed from above (from the animal pole), the third layer rotates back 45 degrees

Variations exist on these two themes with so-called
aberrant cleavage which is a highly modified form of
radial or spiral cleavage which deviates from the given rules. In the annelid worms, polychaetes follow
spiral cleavage, but oligochaetes like the earthworm follow aberrant cleavage which is, nevertheless,
clearly derived from spiral cleavage. In the nematodes (round worms), cleavage is so aberrant that it is
virtually impossible to determine whether it is derived from radial or spiral cleavage. However, rotifers
('wheel animalicules', microscope more-or-less worm-like animals with lobes of cilia that appear to rotate
like wheels) exhibit spiral cleavage and as these are relatives of nematodes, it is reasonable to assume
that in nematodes cleavage is aberrant spiral.

In both radial and spiral cleavage, the early embryo is essentially a solid ball of cells, called a
but already by about the 8-cell stage a hollow cavity begins to appear in the centre of the morula and it
becomes a hollow ball of cells, called a