Simple 2D cell array appending

I have a 2D cell array. I want to do the following:

y = some_number;
row(x) = [row(x)  another_row(y)];

However, row(x) isn't defined until this happens so it doesn't work! How do I simply append another_row(y) onto row(x) when row(x) may not be defined?

Sorry this is easy to do in other languages but I'm not sure how in MATLAB!

Thank you.