A more complete picture of embryonic development

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

• The most revealing time is the first few days, but to observe these stages you need eggs that have just been fertilized. This is best done when the eggs are fertilized in the laboratory at a defined time, not going down to the beach and collecting eggs after dinner.
• The biggest problem in microscopy of living tissue is not usually magnification, but contrast. Obtaining good contrast requires sophisticated lighting and special optics, e.g. phase contrast. These systems cost money and require skill to use.
• The complete story requires many techniques; staining embryos with dyes (which will kill the embryo); embedding the egg and making slices (sections) which are then observed in the microscope; looking at eggs with a scanning electron microscope, etc.

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:

• Embryonic development of all vertebrates (animals with backbones) is very similar, after all they all have the same ancestors don't they? Thus if you find a description of the development of any vertebrate (the chicken is perhaps the most studied) it will be similar to the development of herring.
• There are about ten animals and plants that are considered "model organisms", because have been selected by scientists for practical reasons related to ease of doing research, and have been studied in great detail. We are lucky that the Zebra fish, which is very close to the herring, is one of these. Thus you only need to Google for "embryology of Zebra fish". An example hit is: http://zfin.org/zf_info/zfbook/stages/stages.html

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:

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