By popular request, here’s a roughly annotated version of that zebrafish development movie.
Stuff to watch for:
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This movie starts at the 8-16 cell stage. The cells of the embryo proper (blastomeres) are at the top, sitting on a large yolk cell.
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The pulsing is caused by the synchronous early divisions of all the cells. They lose synchrony at the mid-blastula transition.
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Epiboly is the process by which the cells migrate downward over the yolk. An arrow will briefly flash, pointing to about 11:00, in the direction of the animal pole (where the future nose will form, sorta). That happens just before the whole animal begins to rotate within the chorion, just to make following everything more difficult.
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After the animal rolls over, the animal pole is pointing straight up at you, and the migrating cells will form the germ ring, a thickening around the equator of the embryo. Cells will also migrate towards one point along the ring, forming a thickening called the keel. This is where the embryonic axis is forming; cells are migrating into the interior at this point in the process called gastrulation, and this region is roughly equivalent to the dorsal lip of a frog.
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The whole animal is going to roll over again, this time to its side. The keel is thickening and lengthening towards the animal pole. The body of the fish is going to form along the right side of spherical embryo in this view.
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While the keel is extending anteriorly, cells are still also migrating to surround the yolk—epiboly continues, with the yolk bulging out a bit until it is finally surrounded and closed off at the blastopore.
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The head and tail extend. You’ll see the eye forming, so you’ll be able to tell which end is the head end.
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Along the right side, you’ll also see the tissue form regular little blocks: these are the somites, or body segments.
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The tail continues to extend and lifts off the surface of the yolk. When there are about 18 somites (the resolution is too low, so don’t try to count them), the animal will begin to twitch.
I’ll load up another one in a bit that will show a hint of the horrible stuff we do to them in the lab: we get the babies drunk and watch deformities develop.
