Model Organisms
Large eggs  Physical manipulation (eg injections and transplantations)
Clear eggs  Live Observation
External Development  live observation and physical manipulation
Egg production throughout the year  experiment at any time
Production of a large number of eggs
This is so you can use multiple embryos for many experiments. Eg, take the head from one embryo and put it on the other embryo. However we can not publish this without a control group hence still need more embryos
We have to prove that not just poking the embryo makes two heads, but it is really the transplantation that does it
Thus multiple embryos for reproduction of results before publication
What type of control would you do for a two headed embryo to determine whether poking it or transplantation produces 2 heads?
1) make sure that it is not the MEDIUM you place the embryo into that produces the 2 heads. So one control is just leave the embryo by doing nothing in the medium and one without
2) Another to test whether Poking the embryo produces the 2 heads is to have one control where you do not poke the embryo
Rapid embryonic development then you get faster results.
Short life cycle. A life cycle is from egg  adult or from egg  sexually mature adult (aka 1 generation can be considered a life cycle). A short life cycle lets us use the organism for genetics and such because we can do crosses over several generations in a short period of time
Small organisms with cheap and easy maintenance and breeding in the laboratory

Chicken
Chicken eggs are a large size but however are not transparent. However they can still be useful as because it is possible to cut a small sterile window into the egg and observe it under the microscope and watch the development of the embryo. There are large blood vessels around the yolk which is used to grab nutrients. They also extend towards the shell which allows for CO2 and O2 exchange letting it breathe.
This chicken embryo produces a large amount of blood vessels in order to grow
Xenopus
Lay hundreds of eggs 1.2 mm
Their eggs are not transparent however they develop externally allowing us to observe them. Development is rapid within 4 days.
The life cycle is about 2 months but takes up to really 6 months before sexual maturity
The frogs themselves are large but are cheap and easy to feed
We can use them to do genetic crosses and make mutants, usually in their early development (but there is no transmission genetics)
VIDEO
In this particular experiment the scientists is using an eyelash tool (wtf) to cut off a little bit of the egg embryo tissue from the frog and transplanting it onto another one. And the result is an embryo with 2 heads (WOW)
C.elegans
The eggs are not used that much because they are so small but you can still cut cells with a laser and see how the embryo responds.
They lay a lot of eggs which are transparent
There is very rapid external development. Only 15 hours needed
The full life cycle is 3 days, this allows us to keep it on petri dishes (just put bacteria on the dish and then throw worms on it and let them grow)
There is very sophisticated genetic manipulation for this which has let us understand a lot about developmental mechanisms and the genetics underlaying them

Both C.elegans and Xenopus have cleavage divisions in early embryonic development. These cells are called blastomeres. These eggs have a shell around them which actually confines the embryo growth until embryogenesis is completed and a larvae hatches from the egg.
The term Blast refers to cells that are dividing. These are very rapid and happen about every 20 minutes. Every 20 min the cells undergo cleavage divison.
ANOTHER VIDEO SHOWING THE EGG DEVELOPMENT OF XENOPUS AND C.ELEGANS WOW SO COOL

Xenopus Eggs (A closer look)
There are 2 colors in the eggs, a dark side and a light side. When a frog lays these eggs, each egg has a different orientation. In some cases the light side is up, in