Fertilization and Early Embryology of Starfish.

MATERIALS NEEDED:
§         Prepared slide: Whole mounts and cross sections for stages of starfish embryology.
§         Compound microscope.

    Eggs are classified according to their distribution of yolk. Those with a small amount of yolk uniformly distributed through their cytoplasm are isolecital eggs. When the egg divides, the cleavage plane passes through the entire embryo (holoblastic cleavage). Telolececithal eggs have a large amount of yolk (50-90% of the egg's mass may be in yolk). Telolececithal eggs are divided into two classes, according to the type of cleavage. Those with a moderate amount of yolk (such as frogs) undergo holoblastic cleavage. If there is a large amount of yolk (as in chicken eggs), the cleavage plane does not pass entirely through the embryo. This partial division is called meroblastic cleavage. Centrolecithal eggs have the nucleus and cytoplasm located at the center of the egg and surrounded by yolk. This type of egg is typical of insects.

     The eggs you will examine in this section of the laboratory are isolecital and holoblastic. This type of cleavage is un-complicated by the presence of yolk and demonstrates patterns of development more clearly than do eggs with more yolk.  

PROCEDURE:
1- Obtain a slide with the collected stages of starfish development and observe with your compound microscope. Using figure 5 as a guide, identify the following stages (sketch a representative of each in the results section.)

Unfertilized eggs are recognized by the presence of a nucleus. Unfertilized eggs are haploid in chromosome number. Fertilized eggs (Zygotes) have united with sperm and are now diploid. The nuclear membrane has disappeared and the egg has made a fertilization membrane. The fertilization membrane  is a  barrier to keep additional sperm from entering the egg (why is this important?).

Find an egg that has divided into 2, 4, and 8 cells. The individual cells are now termed "blastomeres". When the cells have  divided four times (16 blastomeres), the embryo has reached the morula  stage. Compare the size of the 2, 4, 8, and morula stages to that of an undivided egg. What do you think caused these results?

Eventually, the mass of cells becomes a hollow ball or  blastula (around the 32 cell stage). At first, the blastula is non-motile. Later in development the blastula acquires mobility. The central, fluid-filled area is the blastocoel. Examine a cross section of a blastula for a better view of the blastocoel.  While the blastula is a single layer of cells, the process of gastrulation produces a gastrula with three cell layers: an inner endoderm, an outer ectoderm, and a middle mesoderm. Each of these germinal layers are important to later development. The endoderm, for example, develops into the lining of the gut while the ectoderm becomes part of the skin and nervous system. The mesoderm develops into muscle, and some internal organs (you will not be able to differentiate the mesodermal tissue on your slides). Gastrulation begins when blastomeres on the surface of the blastula begin to migrate into the blastocoel (much like a balloon's skin follows your finger when you push it into the side of a balloon). The resulting hole in the side of the embryo is the blastopore while the new inner cavity (surrounded by endodermal tissue) is the archenteron (see Box 23.1 for a diagram). The blastopore becomes either the animal's anus or mouth (depending on the species), while the archenteron develops into the gut. The blastopore becomes the mouth in animals such as insects (they are protostomatous- "first mouth". In animals like us the blastopore forms the anus and the mouth is formed secondarily (deuterostomatous- "second mouth"). Examine whole mounts and cross sections of a gastrula. Locate the blastopore, archenteron, ectoderm, and endoderm. Label each on your diagram.

The bipinnaria larva is a motile and feeding stage of  starfish development.

Very bad animation showing how a rotated gastrula can change