Compared with protists,
represent a huge change in the organization of animals: they are
multicellular. While this is obvious, it is worth pausing to reflect
on what multicellularity accomplishes for both an adult organism,
as well as the embryo. Multicelluarity allows for several things
(you could probably add to this list, but these are some of the
most salient features of multicelluarity):
- Regionalization: The development of "differences"
in various spatially distinct regions of the embryo. For right
now we'll remain agnostic about the mechanisms by which this
might happen, with the exception of the case discussed below
- Cell-cell communication: Now that there are mutliple cells,
substances are no longer free to exert effects throughout the
organism if they are present in the cytoplasm, since cells do
NOT share a common cytoplasm. Cells now have the option of communicating
with one another to exert effects on one another, e.g., they
could send signals of various sorts to their neighbors. Induction is one sort of
such signalling exchange.
- Differential cellular movement: Now cells can move from their
original locations, assuming they have some motile appendage
for doing this. Typically though, there is an antagonism between
cell division and cell movement, since both activities requir
large investments in the major cytoskeletal systems of the cells.
- Differential compartmentalization of the oocyte cytoplasm:
If the cytoplasm of a fertilized egg is compartmentalized in
some way, then cleavage results in the differential "parcelling-out"
of such regions of the cytoplasm to various daughter cells in
the zygote. If such regions contain substances that propel the
cells that inherit them toward a particular pathway of differeniatation
(i.e., a cytoplasmic
determinant), then cleavage provides a way to differentially
distribute such determinants.
Food for thought: Can you think of a region of the fertilized
amphibian egg that if inherited by certain cells during cleavage,
might impart different properties on the cells that inherit them?