Protostome-Deuterostome Relationships

A.  Formation of the mouth and anus in invertebrate embryos
     1.  After gastrulation, if the first opening, the blastopore
          becomes the mouth, the animal is called a protostome
     2.  If after gastrulation, the first opening becomes the anus
          the animal belongs to the deuterostomes
B.  Direction in which cells are thrown off at the third cleavage
     stage.
     1.  First two divisions are vertical, the third is horizontal.
     2.  At the third division, the cells towards the animal pole
          are somewhat smaller than those at the vegetal pole and                     
          are called micromeres as contrasted with macromeres.               
          This is because the material towards the animal pole 
          has lest "yolky" material in it and is able to divide more
          completely
     3.  If the micromeres lie directly on top of the the 
          macromeres, the pattern of cleavage is called radial.
          If the micromeres are thrown off at an angle to the 
          macromeres, the pattern is called spiral
C.  Fate of the cells after the thrid cleavage stage
     1.  Cells can be followed through the embryologic development
          of an organism, by the use of vital stains.  Vital stains
          do not kill the cell, but identify subsequent cells in the
          lineage because they will all contain some of the stain.
     2.  In an organism with radial cleavage any of the eight 
          blastomeres can become a complete organism.  Therefore,
          we say at this stage, that the pattern of development is
          indeterminant.  In contrast, organisms that show spiral
          cleavage at the eight-celled  stage, the future of each 
          cell is specified by its particular location.  Eight of
          the eight cells is given a label, eg., the four macromeres
          are A, B, C, and D; the four micromeres a, b, c, and d.
          The D cell becomes the mesoderm.
D.  Formation of the coelomic space
     1.  If the coelom forms by outpocketings from the gut, it is 
          call enterocoelous.
     2.  If the coelom forms by a split in a mass of mesoderm, it
          is called schizocoelous.
E.  Protostomates
     1.  The protostomates for the most part are creatures that have
          the blastopore that becomes the mouth, they have spiral
          cleavage, and the fate of the cells is determinant.  The
          coelomic space if present, is derived from a split in the
          mesoderm.
     2.  The groups belonging to this category include:  annelids,
          arthropods and mollusks.
     3.  The organisms tend to have the different fluid 
          compartments, blood, lymph and coelomic, mixed because
          their circulatory systems are open systems rather than
          closed.  Thus the fluid is actually call hemocoelic fluid
          rather than blood.
F.  Deuterostomates
     1.  The deuterostomates for the most part are creatures           
          where the blastopore becomes the anus, the pattern of
          cleavage is radial, and the fate of the cells in the
          early embryo is indeterminant.  The coelomic space is
          derived from an outpocketing of the primitive guts.
          Typically three pairs of coeloms are formed, the procoels,
          the mesocoels and the metacoel.  Each pair may be 
          transformed into a different component, eg, body cavity
          circulatory system, water vascular system, etc.