The Questions of Developmental Biology

This occurred at precisely the time that epiboly would have taken place during normal
gastrulation. Thus, epiboly may be (at least in some respects) independent of the cells that form
the migrating region.
Axis Formation
Some of the most important phenomena
in development concern the formation of
embryonic axes (Figure 8.7). Embryos
must develop three very important axes
that are the foundations of the body: the
anterior-posterior axis, the dorsalventral
axis, and the right-left axis.
The anterior-posterior (or
anteroposterior) axis is the line
extending from head to tail (or mouth to
anus in those organisms that lack heads
and tails).
The dorsal-ventral (dorsoventral) axis is the
line extending from back (dorsum) to belly (ventrum).
For instance, in vertebrates, the neural tube is a dorsal
structure. In insects, the neural cord is a ventral
structure. The right-left axis is a line between the two
lateral sides of the body. Although we may look
symmetrical, recall that in most of us, the heart and liver
are in the left half of the body only. Somehow, the
embryo knows that some organs go on one side and
other organs go on the other.
In this chapter, we will look at how four selected invertebrates a sea urchin, a tunicate,
a snail, and a nematode undergo cleavage, gastrulation, axis specification, and cell fate
determination. These four invertebrates were chosen because they have been important model
systems for developmental biologists. In other words, they can be studied easily in the laboratory,
and they have special properties that allow their mechanisms of development to be readily
observed. They also represent a wide variety of cleavage types, patterns of gastrulation, and ways
of specifying axes and cell fates.*
*However, model systems by their very ability to develop in the laboratory preclude our asking certain questions
concerning the relationship of development to its habitat. These questions will be addressed in Chapter 21
The Early Development of Sea Urchins
Cleavage in Sea Urchins
Sea urchins exhibit radial holoblastic cleavage. The first and second cleavages are both
meridional and are perpendicular to each other. That is to say, the cleavage furrows pass through
the animal and vegetal poles. The third cleavage is equatorial, perpendicular to the first two
cleavage planes, and separates the animal and vegetal hemispheres from one another (Figures 8.8
and 8.9). The fourth cleavage, however, is very different from the first three.