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EPIGENESIS
A Word Derived From Embryology
The root meaning of the first syllable “epi” is “on top of” as
in epidermis, (the skin on top of the main skin, or dermis)
or epiphyte (a plant growing on top of a plant).
The root meaning of genesis is as in creation, we still use that form. So epigenesis is creation on top of creation.
The word was coined in the study of embryology to describe the processes that take place, each influencing the other, in the development of an embryo.
I’ll try to explain with a few simple diagrams:
The licorice fern is an epiphyte, not a parasite.
Epigenesis is not Epigenetics
Epigenetics is that which is “on top of” genetics, that is whatever is included in the hereditary process that is not part of the DNA based genome. As geneticists develop their field they are finding that there are many other relevant starting points and controlling mechanisms than what we have traditionally associated with the genome.
Epigenesis is a general form process, applicable to many things besides the illustrative example of embryology. Epigenetics is a field of study associated with genetics.
Urchin, Frog, Bird and Mammal
The drawings I was making were for a simple egg, as in a sea urchin. Frog eggs contain more yolk, so the two “poles” of the egg and the developing embryo are markedly different and the blastula cavity is lopsided. The folding-in happens alond the the yolk mass, see the videos below. For bird eggs, with a whole lot of yolk, the embryo develops more like a flat disc than a ball. The same is true for mammals, including human early embryonic development.
There are many resources on the web for human development, but the point I’m making here, namely about the process of epigenesis, is better illustrated with the simpler eggs.
From: LIFE: The Science of Biology, Purves et al 1999, found on: Kenyon Academic Partnership Website
Gastrulation
This video by Alina Korbut shows the folding-in process of gastrulation both photographically and as an animation.
Interactive Epigenesis
I talked about how the neural tube creates swellings that trigger a lens which in turn triggers the eye to form. Here is a drawing of a cross section of this development. It is a typical example of what is spoken of as “induction” in the development field... one thing induces another to happen; reciprocally.
Wikimedia Commons
Optional Internet Resources
1) Frog embryo development video based on photos; shows the early stages of division from single to multiple cells very nicely.
2) Photos and an animation of frog embryo gastrulation that shows the formation of different cell layers nicely.
3) Animation of early human embryo with French subtitles, remarkable in how it shows migration of cells and the formation of the neural tube as an cumulative, epigenic process.