A more complete picture of embryonic development

My amateur microscopic images reveal only a tiny part of this process through a lens darkly:

Not to worry, you can learn more about herring development than you probably want from the Web, a book in a public library, or by even purchasing a book, because:

  1. The egg before a sperm enters to add the second set of chromosomes and start development.
  2. Yoke free cytoplasm accumulates around the entry point of the sperm, and a complete nucleus forms containing maternal and paternal DNA.
  3. The DNA replicates and two cells are formed.
  4. Replication of DNA and formation of more cells continues: 1, 2, 4, 8, 16, 32... The cells form a ball, the blastula.
  5. Dividing cells form a sheet over the yoke mass, and the blastula invaginates, eventually forming the three major cell layers of the adult: ectoderm, mesoderm, endoderm.
  6. The cell layers thicken to form a crescent around the yoke mass and a constriction forms in the middle of the resent. This is the start of a division progression which will create a series of somites. The somites mature into the vertebrae and associated ribs of the spinal cord of the fish.
  7. Now the black pigment in the retina of the eye has formed and thus we recognize the embryo as a fish.

Here is my attempt to draw and describe fish development:

<back to the main herring page>