Although much key research has used Japanese Quail as a source of cells to be grafted to chicken embryos. The nuclei of Japanese Quail cells look very different is stained microscope sections; so you can tell them apart easily. (Also, labeled antibodies are available that bind specifically to quail cells, although this really can't be as trustworthy as nuclear shape, in my opinion.)

Quail-chick grafts became the "gold standard" for fate mapping and was used to prove the neural crest origin of lots of specific differentiated cell types.

Advantages of chicken embryos for embryological research:

1) Cheap! Cheep! Cheep! & Available from agriculture.

2) Fairly large embryos: good source of cells for tissue culture

3) Development can be synchronized by cooling, then warming. In the wild, hens hide their eggs; then begin incubation of many eggs at the same time; so as not to have to care for different ages

4) Some genetic lines.(but these are often lots of trouble to maintain)

5) Chorioallantoic grafting : Isolated organs will develop using blood supply from allantois.

Disadvantages of chicken embryos.

1) Embryos reach ~20,000 cell stage inside hen's body! Making it very difficult to study cleavage or blastula stages.

2) Shell is opaque (but you can cut holes!) (but there are methods by which you can get development of isolated embryos, in glass dishes instead of inside the shell)

The four extraembryonic membranes of Reptiles, Birds and Mammals:

The chorion (furthest on the outside of the 4)

The amnion (="bag of waters" directly surrounding the embryo)

The yolk sac (containing guess what!? at least in the embryos of birds and reptiles)
         (mammal embryos do have a yolk sac, but it isn't full of yolk)

The allantois (connected to the bladder, and storing urine; and also providing major blood vessels to the placenta)

All these are thought to have evolved originally in egg-laying reptiles, for the purpose of nutrition, excretion and hydration. But many snakes are live-bearing (produce live young, without laying any eggs) and many dinosaurs must have been live-bearers.

Julian Lombardi (who was a Lecturer in this department, and then was a Professor at UNC-G, and now I don't know where he is) proposed the hypothesis that these four extraembryonic membranes first evolved in (now extinct) live-bearing reptiles to serve as the equivalent of a placenta; and that their use inside eggs evolved later. Nobody can figure out how to prove or disprove this theory.

Embryonic development of bird embryos occurs by cleavage of the small cytoplasmic layer produces sheet of cells, forming a "blastodisc" instead of "blastula"

The cells of this blastodisc rearrange to form epiblast and hypoblast.

But all of the eventual body develops from epiblast; all the original hypoblast cells become part of the yolk sac.

Lateral edges of the epiblast crawl actively (like little tractors) outward across the inside surface of the vitelline membrane. This epiboly eventually surrounds the whole yolk.

The blastodisc center becomes the "area pellucida " (which is transparent) This is surrounded by the "area opaca" (yolk still in cytoplasm)

A thickening called the "primitive streak " elongates from one edge inward toward the center. (from whichever side is up i.e. gravity controls which direction A-P axis forms.

But if a blastodisc is cut in 4 pieces, each piece can develop an entire primitive streak and a separate embryo.

Gastrulation along primitive streak, with Hensen's node, etc. internalization of future mesoderm and endoderm.

Waddington discovered that transplantation of the Hensen's node will induce an entire second embryo. (it is equivalent to the dorsal lip of the blastopore) It is also the site of internalization of the future notochord cells.

Development of mammal embryos

Marsupials (like Opossums, and the many kinds of Kangaroos, etc.) have oocytes that are much larger & yolkier than higher mammals & I really don't know what their gastrulation is like!

The fetuses crawl from the womb to the pouch, and begin nursing at a very early stage of development.

This has helped to permit research on limb bud development and limb regeneration, and other events of later development.

The great majority of mammals are Placental Mammals .

They have small oocytes, about 100 micrometers in diameter. (very similar to sea urchin eggs!)

They also have a tough, but transparent vitelline membrane (called the "Zona Pellucida" )

They have slow, non-synchronous, cleavage cell cycle time 20-24 + hours in man Cells do not even adhere very much to each other at first.

As late as morula stage, mammal embryo cells will intermix freely with morula-stage embryos of other mammals species, even mouse with rat, and sheep embryo cells with goat embryo cells.

The resulting animals are called chimeras and are relatively easy to make & will develop if implanted into the womb of a large enough mother.

Probably we should break this news slowly to congress, if at all!

Next comes the time of compaction .
At this time mammal embryo cells become much more adhesive to each other.

They synthesize lots of cell-cell adhesion protein (one of the cadherins )

Also secrete enzymes to digest their way out of the zona pellucida

By this time they should have passed down the upper oviduct (Fallopian tube ) into the uterus where they undergo implantation in the endothelium (special epithelium that lines the uterus)

But they can also implant in the wall of the Fallopian tubes

The result is an "ectopic pregnancy" = '"tubal pregnancy "

The cells of the trophoblast becomes part of the chorion of the placenta

" inner cell mass = the "stem cells that politicians are debating so much, because they can be grown in tissue culture in unlimited amounts and might be able to re-grow damaged organs?

The body develops entirely from cells of the inner cell mass.

Identical twins can be formed by mammals in three different ways:

(but probably never by separation at the two cell stage, because the zona pellucida is so tough)

1) Formation of two separate trophoblasts, each containing its own inner cell mass. (50%)

2) Formation of two inner cell masses within one trophoblast . (50%)

3) (the rarest form, which is only ~~1% of identical twins in humans) Development of two primitive streaks within one inner cell mass. (which is, of course, inside one trophoblast).

In a small fraction of this third class of twinning, the twins are conjoined , which is the same as what used to be called "Siamese twins".

Some part of the bodies of conjoined twins are shared, as if they were fused together.
They can be joined by just about any part of the body, but notice that they are always (I think!?) mirror images of each other.

By observing which of their extraembryonic membranes are shared between identical twins, you don't have to be Sherlock Holmes (or even Dr. Watson) to deduce which of these mechanisms of twinning has occurred in a given case. Sometimes identical twins don't share any of their extraembryonic membranes, so that their placentas are as separate as if the were non-identical twins. In other cases, the twins share the same chorion, but have separate amnions. In other cases, both twins have developed inside the same amnion. Whether they ever share the same yolk sac, I don't know, and wish someone could tell me where to find out.

In a few kinds of mammals, the third type of identical twinning is normal. Armadillos always give birth to identical quadruplets, formed by 4 primitive streaks (4 Hensen's nodes, etc.).