The "Lophotrochozoa" hypothesis is also supported by the fact that many phyla within this group, including annelids, molluscs, nemerteans and flatworms, follow a similar pattern in the fertilized egg's development. When their cells divide <answer>after the 4-cell stage, descendants of these 4 cells form a spiral pattern</answer>. In these phyla the "fates" of the embryo's cells, in other words the roles their descendants will play in the adult animal, are the same and can be predicted from a very early stage. Hence this development pattern is often described as "spiral determinate cleavage".
How do the cells of Lophotrochozoa eggs arrange themselves?
after the 4-cell stage, descendants of these 4 cells form a spiral pattern