Cleavage stages in a representative
 animal species --  the  Sea Urchin
 The ANSWERS

   2.               
    Do you want to first see fertilization of an urchin egg?  This is sexual
    reproduction at a simple level.

     In the early part of this video you see only two eggs.  The urchin sperm are way
    too small to view.  As soon as the sperm penetrates the egg, the egg wants to
    stop any other sperm from entering.  It creates a chemical shield on the outside
    of the egg.  This is the F E R T I L I Z A T I O N   MEMBRANE that appears
    very early in the video.
  

      Here's a geometric doubling series, you may soon need:
      1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048

      a.     In stage "A" just after F E R T I L I Z A T I O N, there is/are _ 1_ cell(s).
        b.    After one cell division (M I T O S I S) at "B", there are now  _ 2  _ cells.
        c.     After two cell divisions at "C", there are now _ 4 _ cells.
        d.     After about four cell divisions at "D", there are now about __ 16 __ cells.
        e.     After about six cell divisions at "E", there are now about __ 64 __ cells. 
        f.     After about eight cell divisions at "F", there are now about __ 256 __ cells.

        Here are some classical steps of the continuous early embryonic life of organisms..
        Remember that cleavage is a continuous process, but we can still study stills.  It's like
        watching a slow motion replay of a sporting event.  The event took place in real time,
        but we are free to study and analyze the individual steps in a continuous play.

 a.       Morula stage looks like a “bunch of  G R A P E S”         (4600)
       But Sea Urchins seem to skip the morula stage of development and move directly
       into the blastula stage..

 b.   Blastula stage  "E" & "F" above (about __ 64 __ - __ 256 __ cells) looks like a
       “H O L L O W   ball of cells”           (5174)

 c.       Gastrula stage (after "F") in which cell  L A Y E R S  start to form and
       the “start of  D I F F E R E N T I A T I O N”   (22395-97)              

        Now let's see if we can find a video on complete early development of this urchin.

    You will see all the developmental  C L E A V A G E  steps after fertilization listed above and
     beyond.  Near the end, you will see activity in which part of the cell layer of the Blastula rotates
     inward creating new tissue layers.  This is the end of cleavage, which means that now after
     mitosis, the daughter cells will start to   G R O W.  The tissues that form will now start to take
     on individual different characteristics.  This is the beginning of  D I F F E R E N T I A T I O N .

     This is what you also saw in the video. 
     
    Firstly, an egg was fertilized and a M E M B R A N E  developed around it.

    Then, the egg underwent  C L E A V A G E  in which the number of cells  I N CREASED,
    while the average size of each cell  D E CREASED. 

    Finally, T I S S U E  layers started to form as the early embryo entered the  G A S T R U L A 
    stage of its development.  .  

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