How to incorporate varying time intervals between repeated measures in a General Linear Model repeated measure covariate analysis in SPSS, HELP!

This may not be particularly helpful, but these guys did a somewhat similar analysis and you may be able to work backwards from how they laid out their graphs to figure out if your variables are in the right place. They used day/degree against length and wet weight. Later they compared dry yolk sac weight against dry larvae weight.

Thanks for that, I have read that paper already. It does mean that it is possible to do somehow but finding the right buttons to click and getting the data in the right places for the stats to work is currently really starting to annoy me!

Could you refer to them as Encounter 1, Encounter 2, Encounter 3 etc and get away from the variable interval lengths and see how they graph?

I can do that but it is the rate of yolk sac absorption I want to look at therefore the varying time intervals are very important. I have plotted a graph with this data using the egg size as a covariate and with the time increments all assumed to be equal but this affects the curvature of the graph and so does not have a true representation of what is happening (therefore I wont be able to use it in my thesis).