The Questions of Developmental Biology

eggs.
Coda
This chapter has described early embryonic development in four invertebrate species,
each of which develops in a different pattern. The largest group of animals on this planet,
however, is another invertebrate group the insects.
We probably know more about the development of one particular insect, Drosophila
melanogaster, than any other organism. The next chapter details the early development of this
particularly well-studied creature.
Snapshot Summary: Early Invertebrate Development
1. During cleavage, most cells do not grow. Rather, the volume of the oocyte is cleaved into
numerous cells. The major exceptions to this rule are mammals.
2. The blastomere cell cycle is governed by the synthesis and degradation of cyclin. Cyclin
synthesis promotes the formation of MPF, and MPF promotes mitosis. Degradation of cyclin
brings the cell back to the S phase. The G phases are added at the midblastula transition.
3. "Blast" vocabulary: A blastomere is a cell derived from cleavage in an early embryo. A
blastula is an embryonic structure composed of blastomeres. The cavity in the blastula is the
blastocoel. If the blastula lacks a blastocoel, it is a stereo blastula. A mammalian blastula is called
a blastocyst (in Chapter 11), and the invagination where gastrulation begins is the blastopore.
4. The movements of gastrulation include invagination, involution, ingression, delamination, and
epiboly.
5. Three axes are the foundations of the body: the anterior-posterior axis (head to tail or mouth to
anus), the dorsal-ventral axis (back to belly), and the right-left axis (between the two lateral sides
of the body).
6. In all four invertebrates described here, cleavage is holoblastic. In the sea urchin, cleavage is
radial; in the snail, spiral; in the tunicate, bilateral; and in the nematode, rotational.
7. In the tunicate, snail, and nematode, gastrulation occurs when there are relatively few cells, and
the blastopore becomes the mouth. This is the protostome mode of gastrulation.
8. Body axes in these species are established in different ways. In some, such as the sea urchin
and tunicate, the axes are established at fertilization through determinants in the egg cytoplasm.
In other species, such as the nematode and snail, the axes are established by cell interactions later
in development.
9. In the sea urchin, gastrulation occurs only after thousands of cells have formed, and the
blastopore becomes the anus. This is the deuterostome mode of gastrulation, and is characteristic
only of echinoderms and chordates.
10. In sea urchins, cell fates are determined by signaling. The micromeres constitute a major
signaling center. β-catenin is important for the inducing capacity of the micromeres.