This should be a fun one! I'm a little excited to be challenged like this! I'm pretty confident in how to make the sperm and egg but my concern lies with the petri dish.
Depending on the size cake the lady wants, I'll bake it 4" high round (I'm thinking 12" round is best) then dig out the middle, leaving the sides. Cover in white fondant but how do I make it appear see-through?
What dust/water/vodka might help make it appear translucent?
How are you with poured sugar? I'm thinking do a base cake and then make the petri dish from Isomalt with the sperm and egg in it. But, otherwise, I can't think of a way to make the "dish" appear translucent. But, of course, I haven't had a full pot o coffee yet! LOL
Good Luck!
Beth in KY
oooh, never tried poured sugar. I'm not sure how I would be with it.
I was thinking about just making a 3D egg and sperm, having it sit in the middle of the cake
but
maybe that would look weird. Because then it wouldn't look like it was a real petri dish. Man, now I'm really not sure about this cake LOL 
As far as I know, there's nothing you can do with cake to make it appear translucent. 
I remember seeing an episode of Ace of Cakes where they were discussing this subject and they just used gumpaste to make cups and painted the outside to look like what was in the glass. If Duff doesn't know, I certainly don't!!
The only thing I can think of would be pulled sugar. Maybe if you made the round cake and made a pulled sugar (or poured sugar) strip to wrap around the outside of the cake?! I'm not sure if that would work because i've never worked with poured/pulled sugar but there is a forum on CC. You could post a question there. Good Luck! 
Sorry, I guess I took too long to write. Seems Bethola and I are on the same wavelength!
i dont know about the whole egg/ sperm things, but usually when petri dishes are used, theres a layer of jelly in them. i was going to through that out there to see it it helpd anyone else but...
would it be possible to make a sheet of jelly, to wrap around the cake? if you had a plain jelly, then it would be more see through too. maybe an idea if you dont fancy the poured sugar thing.
xx
good idea about the jelly....
Right now, my plan is to make 2 -12" rounds that are 2" high. Stack on top of each other. Take my 10" round pan and trace it out on the top of the 12". Cut out that 10" circle of cake to the top of the bottom 12".
Cover sides, top, and just the inside with white fondant. Cut a 10" circle of white fondant and lay that in the inside of the petri dish. That way, the fondant is pretty seamless. I can always pipe a white strip of royal icing along the inner edge where the circle of fondant meets the other fondant that covers the outside, top, and inner sides.
Make a gumpaste sperm. Easy.
Make a cakeball egg. I've made cake balls in the past where there was too much icing. I used my ice cream scoop and when I scooped it out and plopped it down on my wax paper, it looked JUST like the texture of ice cream and held it's shape well. I have a few pictures of the 'moment of conception' that i think this technique would work well for.
Maybe put piping gel down on the inside of the petri dish for the 'jelly' and then lay down my cake ball Egg and have my sperm swimming towards it.
Now, i'm waiting on an email from the lady to decide if she wants many sperm fertilizing the egg, or just one swimming towards it, or getting ready to fertilize it.
These questions I never thought I would have to ponder hahaha
at least it sounds like a fun cake to be challenged with!
xx
I am a biomedical scientist turned stay-at-home-mom with a lot of actual petri dish experience. In reality, a sperm and egg would likely be placed in liquid media that would likely be hot pink or red in color. The "jelly" or agar is used for growing bacterial cultures, etc, not things that swim around. Just my two cents. Sorry this probably isn't too helpful for you, I was just excited to talk science and cake at the same time!!
What about some type of heavy jello? Just tossing it out there! 
On one of the cake challenges on Food Network - the winner was a santa jack in the box....they were making a glass of milk to sit with the cookies...and they made it as a clear glass from gelatin.....not that I'm sure it would work because they had a lot of trouble with it on the show....maybe someone else remembers that episode and the name of the baker and you could do some internet research.
There is a product called unbreakable gel. I have not used it but when I wanted to make a bubble for my Buzz Lite year character this is what people reccomended.
Get yourself some acetate sheet and cut out templates of the dish bottom and side.
Bloom some gelatin (about 1 measure gelatin to 2 measures water) in a heat-proof container for about 5 minutes. Then place the container in a hot water bath until the gelatin melts completely.
Shape your side template with tape. Paint several coats of gelatin on the inside, allowing to dry completely before adding next coat. Do the same with the bottom (this takes time to accomplish, so don't get frustrated. And the first coats will appear thin when dry, so don't be alarmed when it seems like it has disappeared).
When you have at least 10 coats on each, arrange the side template on the bottom template so the edges match up. Then add a few more coats, filling in the edge where the side and bottom meet.
Allow to dry overnight. Remove template when you feel it is thick enough for you to work with.
It's inexpensive but a bit time consuming. But the gelatin will dry very clear, and you don't have to worry about burning yourself with hot sugar.
Theresa 
PULEEZE take a pic of this finished cake i'm dying to see the results (i think jello fruitsalad meets sperm(s) and egg as seen from the top of the petridish?) the cake's the dish and the middle the liquid? i honestly am confused (i've said this more than once today too) but what about clear gel-piping gel? well good luck!!
WOW playingwithsugar thanks for the tutorial...i might need to try that when i have some time. I'm a little crunched this week, though.
I will most definitely post pictures.
I got more info from the lady. She likes my plan and is not worried about it looking realistic. In fact, she wants it to be cartoony, complete with happy smiling faces on the sperm and egg
yes, i think i am going to use some piping gel in the center of the petri dish...probably a light blue, maybe?
I love it! It's got great humor without being crass. Great job!
, That is SOOOO cute. It is so neat how you came up the something so cute from an unexpected theme. Great Work 
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