When an egg is layed, how does the flech, blood, bones etc form? Are they formed from scratch? or is the existing material inside the shell is the flesh and bone but just in the right shape yet?
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I’m seeing a lot of misinformation here, but to be fair, ELI5 is pretty challenging for this concept. The embryo in the egg starts as a single diploid cell formed by the union of two haploid cells (haploid cells are gametes with half the genetic material required for the animal to develop normally, while a diploid cell has all the necessary genetic material). That single cell begins dividing into exact copies (embryonic stem cells) until it reaches a threshold (lets arbitrarily say 500 cells). Then the cells begin to differentiate into cells that become the gut, skeleton, muscles, skin, and nerve cells. These differentiations are terminal and cannot go back to a stem-like state. All the while, these cells need nutrients (yolk and albumin) that are absorbed by diffusion.
There is actually a fantastic experiment common for college embryology labs where the contents of the fertilized egg are removed and transferred to a petri dish so development can be observed over time. After cell differentiation, cells from the eye or other organs can be transplanted to different areas and will continue to grow. That experiment is an example of how after cells terminally differentiate due to exogenous signaling, they are “destined” to form a particular tissue or organ.
Hopefully somebody can distill this down to a ELI5 level, because I know I totally failed at that.
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