Lets suppose that the blastaea lives in a gravitational field, probably the sea. No matter what its overall density we can bet that not all the cytoplasmic contents of the blastaea are equally dense, oils and fats will be lighter than proteins and carbohydrates. In an egg cell, a gamete, they will tend to layer out. So the egg could be the source of pattern and pass it on to the embryonic cells. Or the blastaea itself could acquire pattern from the environment in another way.
Air is a discouraging medium for a marine animal, so lets suppose it sinks.
The end which hits the mud might stick to it - to form a sedentary adult. Or it might invaginate, because it is stimulated to divide, or it might die, or restrict reproduction to the non-mud end. In any case, an axis has been determined and a pattern set up. We have a slight problem here. We could say that gravity, the signal, follows the ideas of Baldwin and Lamark and becomes incorporated into the genome.