The metazoa are distinguished from the protozoa by their multicellular organization. Embryogeny, the process by which multicellular development occurs, is the growth and layering of specialized, interdependent, coordinated cells. These germ layers, or embryonic tissues, provide the framework for the complete development of the metazoan animal.

Beginning with egg, there is some variability in the embryogeny of invertebrates. Eggs can be described by their amont of yolk and its placement as isolethical (small amount evenly distributed), telolecithal (yolk at one end/pole), or centrolecithal (yolk concentrated in center). Once the sperm penetrates the egg and fuses, a zygote develops initially through a process called cleavage. Cell division results in the production of blastomeres that are completely separate (holoblastic cleavage) or partially separated (meroblastic cleavage). The orientation of the blastomeres can also be described directionally by radial cleavage or spiral cleavage, refering to how the subsequent divisions took place. Finally, an organism can be classified by how early the ìfateî of the cells are determined. In indeterminate cleavage, the cells ìfateî is not fixed until after the 4-cell, or possible later, stage. In this case, each separate cell can be removed individually and they will develop normally. Determinate cleavage is just the opposite. The function of the cells determined early in development (as early as the 2-cell stage) and removal of any cell will result in abnormal development (the missing cell cannot be replaced). In general, the ìolderî groups of invertebrates (protostomes) exhibit spiral, determinate cleavage, while the ìyoungerî groups (deuterostomes) exhibit radial, indeterminate cleavage, similiar to that of vertebrates.

In discussing the subsequent development of invertebrates, their life cycles can be categorized into three main groups. Indirect development refers to the zygote growing into an aquatic larva that is distinctly different from the adult. As a larva, it will either feed on plankton in the water column (planktotrophic larva) or feed on yolk supplied from the mother (lecithotrophic larva). The larva must then go through a metamorphosis to become an adult. Direct development involves embryos that are cared for by the parents in egg capsules (oviparous), brooded in the mother (viviparous), or brood egg capsules (ovoviviparous). In all cases, the young emerge as juveniles resembling the adults. Finally, there are cases of mixed development that combine parts of the two mentioned above, whereby the embryos are brooded for some time and then released as a larva stage.

In this lab, we will observe embryogeny in sea urchins and life cycle development in insects. Sea urchins, being echinoderms, are deuterostomes and should exhibit radial and indeterminate cleavage. Initial division is a holoblastic cleavage. Winged insects exhibit a mixed development. We will look at mosquitos specifically, which exibit a holometabolous metamorphosis.

I. Sea Urchin Embryology

We will use potassium chloride to stimulate gamete shedding in the purple sea urchin and observe fertilization and embryo development.

a. invert sea urchin over shedding cup so that mouth is up. Inject about 1 cm3 of KC1 into the soft tissue surrounding the mouth. Gametes should be released within 2 - 5 minutes after injection. Eggs will be yellow and sperm will be white.

b. Wash eggs and sperm separately in fresh seawater.

c. Place one drop of eggs on a depression slide and cover with a cover slip. Position the slide under the microscope and observe the appearance of an unfertilized egg. Add a drop of sperm suspension to edge of cover slip. Fertilization can occur within 30 seconds.

Watch for the rising of the membrane surrounding the egg and the formation of a clear layer on the surface. The first division should occur at about 50-60 minutes after fertilization.

d. Observe samples set up earlier that should be in later stages.