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The first 3 cell divisions bisect the
embryo equally. The first 2 cleavage planes from the "top"
(known as the animal pole) to the "bottom" (known as the
vegetal pole), while the third runs acroos the equator and
separates the embryo into "animal" and "vegetal"
halves.
2.
4th cleavage is
asymmetric#8, #9
The 4th cleavage is more unusual.
The cells in the top half divide equally, but those in the
bottom half divide unequally, creating large cells
(macromeres) and small cells (micromeres). This is
accomplished by one of the centrioles positioning itself in
the middle of the cell, so that the spindle is displaced to
one side.
3.
Formation of blastulaLab
Handout, #10 The cells continue
to divide until they form a hollow ball known as the
blastula. Each of the cells contains a cilia. An enzyme is
released that digests a hole in the fertilization envelope
and the embryo begins to swim. Also at this point, the
genome is activated and starts to express new genes.
2.
GastrulationLab
Handout,47.9
a. PMCs migrate and form
spicules (skeleton)#11
b. Vegetal plate forms
archenteron (gut)#12,
#13
c. Basic body plan
d. Pluteus
larva#14
Shortly
after hatching, the descendants of the micromeres at the
vegetal end detach from the epithelial sheet and move into
the blastocoel (ingression). These are known as primary
mesenchyme cells and they form the calcium carbonate
spiculesof the larval
skeleton.
The
remaining cells thicken to form the vegetal plate,
which invaginates to form the
archenteronor gut.
This process is known as gastrulation and, in addition to
forming the gut, it results in
a multilayered body plan. The archenteron extends by cell
rearrangement (see figure) and by connections between the
cells at the archenteron tip (secondary mesenchyme cells)
and the extracellular matrix lining the blastocoel. Once the
archenteron reaches the other side, the mouth is formed. As
the skeleton is laid down, the embryo's shape changes to
form the prism and then pluteus larvae.
(Campbell, 4th, pp.965-
973)
Review
questions
1. Fertilization is the
union of haploid sperm and egg. What are the two functions
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