[DoctorBeaver (OP)]

I've just been watching a BBC iPlayer program about cells http://www.bbc.co.uk/iplayer/episode/b00m6nhq/The_Cell_The_Chemistry_of_Life/. They were talking about the work done by Theordor Boveri on sea urchins, but this questions applies to all ova. It has made me realise there is a gaping hole in my knowledge of cytology.

The program contained the requisite video sequence of cell division within the ovum and that got me wondering. If cell division takes place within the confines of the ovum, what happens to the ovum wall? Does the whole embryo develop within the ovum or does there come a point when it breaks free? I don't see that the wall can become skin as skin is made of cells & the ovum wall isn't.

As a supplemental question, I noticed that the 2 separating halves of the cell pulsated wildly as the chromosomes migrated to opposite ends of the cell and division began. What is going on there?

[Nizzle]

The wall is not made up of cells, it's made up of a secretion of proteins and glycoproteins.
Firstly, the cell division occurs within this 'protein shell' which leads to doubling of # of cells, and the "Mean Cell Volume" halves with each division.
After a while, the protein shell dissolves and massive growth of the embryo's volume can commence.

Since I'm unable to watch the vid here at work (site is blocked), I can't answer the pulsation question

[DoctorBeaver (OP)]

Thanks, Nizzle. I'd guessed it must be something like that.

[wanhafizi]

Trophoblast...

Later, it develops into placenta.

http://en.wikipedia.org/wiki/Trophoblast