Cleavage, Blastulation, Gastrulation & Larval Stages
1. Cleavage in sea urchin involves two concepts
        a. Holoblastic
        b. Radial
2. Holoblastic Cleavage means the whole egg will cleave.  This is not the case for all organisms
3. Radial Cleavage means the that the egg cleaves at right angles to the animal and vegetal pole
        a. Animal Pole – the end where the animal cells come from.
        b. Vegetal Pole – the opposite end where nutrients, such as the yolk, are made for the cells in the animal pole
                to consume for food.
4. First cleavage is about 1 hour at room temp, then subsequent cleavages takes about 30 minutes thereafter.
5. At the 16-cell stage the fertilized egg looks like a morula
6. After 5-6 hours after fertilization the egg is in Blastulation stage and is now called a blastula
7. By 7-8 hours the blastula forms cilia over its entire self.  These cilia beat causing the egg to “swim” around. The
        blastula also has an empty cavity inside called, no surprise, a blastocoel. Remember that the ciliated blastula is a
        hollow ball with a cavity on the inside.
8. Gastrulation after the ciliated blastula stage
9. Primary mesenchyme cells (which started developing around the 16-cell stage on the edge of the inside wall of
        the blastocoel) start to migrate into the center of the blastocoel. These mesechyme cells will secrete the skeletal
        supports of the larva called spicules. [note: this is the same term used with the sponge, remember]
10. The spicules first have three-arms, hence the name tri-partite. These then further branch into several arms.
11. Gastrulation continues by invagination of the blastopore at the vegetal pole to form the archenteron, or primitive gut.
 12. Secondary mesenchyme cells are at the far end (opposite to the blastopore) of the archenteron.  These loose collection
        of secondary mesenchyme cells stretch out filopodia [like how the amoebae moves] to contact the inside of the
        blastocoel and then pull the archenteron towards the other side of the blastocoel.
13. After connecting the archenteron to the blastocoel, these secondary mesenchyme cells later disperse and form
        mesodermal organs.
14. After about 18-20 hours the gastrula becomes prism shaped larva and after a few hours is called a “pluteus larvae”
 

Sea Urchins:

        Fertlilzation    -    Morulation    -    Blastulation      -      Gastrulation
 

Chordata: same, but we have presence of a notochord so we add a step of development
 

        Fertlilzation    -    Morulation    -    Blastulation      -      Gastrulation   -        Neuralation