Below is a summary of the features and structures you should identify in fixed embryos.  Scroll down to see information on Xenopus specifically.

Click here to see figures demonstrating sagittal and transverse planes of section

Cleavage stages
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  Whole embryos

• You can see the jelly coats surrounding these embryos.  You can also see the inner, vitelline membrane.
• Note animal pole pigmentation, vegetal is unpigmented
• The dorsal side of these embryos is in general less pigmented than the ventral side, so when looking down at animal pole, you may be able to pick out the dorsal side (it's pretty subtle).

• Animal half has micromeres, smaller than the yolk filled macromeres of the vegetal half
• Again, you may be able to see that the dorsal side is less pigmented

• The cells at the animal pole are smaller, larger at the vegetal pole.  You canít see the blastocoel from the outside, but it has formed by this stage.  When you cut open your embryos, look for the blastocoel--a space, or cavity within the embryo.

• Note the blastocoel.You may be able to distinguish dorsal from ventral because thereís less pigment on the dorsal side.  Also, the blastocoel is closer to the dorsal surface, so the embryo wall is thinner there.

Gastrula stages
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    Whole embryos

• Try to find the dorsal lipóyou have to roll the egg onto its animal pole.  The dorsal lip is a little pucker on the dorsal-vegetal side, and it is slightly pigmented.  It is difficult to see this structure in early gastrula stages, but at the lateral lip stage, the lip has expanded, and you should be able to see it.  You can also look at a late stage gastrula (yolk plug stage) to see the complete circle formed by the joining of the dorsal and ventral lips. Youíll see a depressed circle with a whitish yolk plug.

• Identify: archenteron, archenteron roof, blastopore, dorsal and ventral lips, blastocoel, and yolk plug.
• At higher power, you should also be able to distinguish ectoderm (outer layer), mesoderm (middle) and endoderm (inner).

Neural Tube
Whole embryos

• Note how the embryo is no longer round.  It's starting to look more like a tadpole.
• You should be able to see the neural folds as they come together, enclosing the neural tube, the ridge that you see along the dorsal edge of the elongating embryo.

• Identify the: notochord (dorsal mesoderm), lateral plate mesoderm, and pharynx.
• Also look for optic and otic vesicles (only visible in some sections)

• Identify the prosencephalon, mesencephalon and rhombencephalon, somites (may not be visible in each slide), pharynx, mid and hindgut, spinal cord, and notochord.

Xenopus Information

Click here for all you ever wanted to know about caring for and handling the african clawed frog, Xenopus

Click here for more scientifically relevant stuff:  a great site with tutorials about cortical rotation, division, gastrulation, etc.
 

During lab, you should observe the following features of Xenopus development.

• differences in pigment between animal and vegetal hemispheres
• changes in pigment during fertilization (this is subtle)--fertilized eggs have pigment covering about 1/3 of the surface, while unfertilized eggs have pigment covering about 1/2 the surface
• once de-jellied, the egg is affected by gravity, and thus the eggs should rotate so that the animal pole faces up.  Eggs that do not rotate are probably not fertilized and will probably not undergo cleavage.
• observe the first cleavages to make sure that the embryos you have selected are actually fertilized and are going to survive.  Notice how the cleavage furrow forms and how it does not completely cleave the vegetal half.
• In a normal embryo, the first cleavage bisects the grey crescent, dividing the embryo into a left and right half.  You may see cleavages that are not directly down the midline in your UV-treated embryos.  It may be interesting to follow these embryos to see what happens to them.