Large yolky eggs, but holoblastic cleavage; cells very big:
embryonic tissues heal very rapidly, up to neurula stage or later.

Size and rapid healing make it practical to do very drastic grafting of tissues. Grafts heal into position in only a few hours

From about 1900-~1960? more embryological research was done using amphibian embryos than in any other group of animals.

Much grafting and regeneration research has continued, now, using newts, axolotls, and Xenopus frogs. Rana frogs are very difficult to raise in the laboratory so they could not be model organisms

Gastrulation will begin is on whichever side is opposite to where the sperm enters.
All 360 degrees of the surface nearer the vegetal pole has the potential ability to form the blastopore, and archenteron, until sperm entry "breaks the symmetry" so that the previous radial symmetry (an infinite number of planes of reflection symmetry), is reduced to one single plane of reflection symmetry

There are many different kinds of symmetry (dozens) and "symmetry breaking" is one of the most important concepts in any subject where geometric patterns are formed.
Eventually, these concepts will become just as important in developmental biology as they are in crystallography etc. .

Often, what most people think of as the development of symmetry is really a reduction of symmetry.
For example, Kartagener's syndrome proves that embryos use the lack of reflection symmetry of flagellar basal bodies to break the reflection symmetry of the body (heart asymmetry, etc.)

Cleavage: the cells formed nearer the animal pole are much smaller (~<1/10th) than those near the vegetal pole, but this is because horizontal cleavage furrows cut cells unequally,
NOT as the textbook says on page 306 that cleavage rates are slower in the more yolky parts of the embryo. All the early cleavages are nearly synchronous.
The mitotic spindles locate toward the less yolky sides of cells.

Sperm entry stimulates a rotational sliding of cortical cytoplasm by about 30 degrees toward whichever side the sperm entered (refer back to photos and diagrams on page 211 of the textbook)

This upward sliding of cortex creates the gray crescent.
Which in some species is actually gray, but in many species is invisible.
But even when invisible, the blastopore will still form there.

Experimenters tried putting sperm exactly on the animal pole, and also tried activating eggs without fertilization! Would the egg be able to decide where to put its blastopore?

The result was that the eggs rotated with respect to gravity,
& this rotation caused formation of a gray crescent & blastopore

So then they tried orienting them straight up and down, and then fertilizing exactly at the animal pole.
But the embryos still formed blastopores "randomly" (or as the textbook says "unpredictable")
which really means we don't know what the causal mechanism is!
(The real key mechanisms of symmetry breaking are probably internal)

If you separate the first two cells of an amphibian embryo (or separate the first 4 cells into two groups of two) then embryos will develop only from those parts that contain some gray crescent

Incidentally, the first cleavage furrow doesn't always bisect the gray crescent; in many species, large percentages of embryos cleave first in the axis perpendicular to the plane of future symmetry contrary to what the textbook says!

If you separate the half of the embryo that doesn't have any of the gray crescent, then it will differentiate into just a blob.

If you wait until early gastrulation and, dissect out the dorsal lip of the blastopore, and then insert this tissue into the blastocoel of a second embryo, then this will differentiate into notochord & somites and will "induce" the surrounding cells to change what cell types they will differentiate into, and form a whole twin.

For example, the host ectoderm over this graft will form a second neural tube
(and the endoderm below it differentiates into a second digestive tract)

This was discovered in her Ph.D. research by Hildegard Proschholdt
and won the Nobel Prize for Hans Spemann years later.

Johannes Holtfreter was also a grad student in Spemann's lab then
and discovered several things, including that no neural tube (at all) will develop if "exogastrulation" occurs, so that none of the ectoderm is in contact with mesoderm.

These discoveries led to a concept of sequences of inductive chemicals, like dominos knocking each other over, in series.
They also led scientists to expect the Nobel Prize would also go to whoever discovered which proteins caused these inductions.

The dorsal lip of the blastopore (= future notochord) was said to be "the organizer" and to cause "primary induction", stimulating adjacent ectoderm to become neural.

Since then, the subject has been crowded with busy climbers.
(but I will try to reduce their results to a few key facts)

Summary of what you should learn from pages 312-338

The Nieuwkoop center: Pieter Nieuwkoop is a Dutch biologist who tissue cultured small pieces of Xenopus blastula stage.
Cells from the bottom became endodermal cell types;
Cells from the top became ectodermal cell types;
Cells from the middle became, guess what?

? ? Cells from the middle sometimes became ectodermal and sometimes became endodermal, but never became mesodermal!?
? What sense does this make?
The conclusion is that induction of mesoderm requires a combination of dorsal and ventral signals. If you mix cells from top with cells from bottom, then some of them differentiate into mesodermal cell types.
The particular ventral cells that induce more dorsal cells to become the organizer have been named the Nieuwkoop center.

Many specific genes are active in this area; such as beta catenin (related to armadillo) which forms mechanical links between actin and membrane adhesion proteins, and is also a transcription factor.

Don't bother to memorize the other genes "cerberus", Dickkopf", Frzb, Chordin, Noggin, goosecoid, Derriere, etc. The one called "Sonic hedgehog" will come up later.

One of the surprising claims is that notochord doesn't really induce ectoderm to become neural; the new version is that something from notochord blocks a factor that induces ectoderm to become skin. So it's the inhibition of the stimulation of the alternative differentiation!
(This is based on effects of a dominant negative activin receptor.)

Some other observations by Holtfreter that are not yet understood

Groups of cells dissected from the dorsal lip of the blastopore will actively burrow into any part of the embryo (shown in drawings on page 313: where it is said to be evidence of larger "surface tensions", but I think it's really caused by stronger apical contraction plus more cell-cell adhesion)

Also, uncleaved eggs try to invaginate at the gray crescent! (a slide of this will be shown in class)