An Introduction to Zoology: Chapter 4

Another characteristic of protostome development is that cleavage is usually determinate. What determinate cleavage means is that the ultimate fate of each cell is determined very early in development. A snail, for instance, is a protostome and has determinate cleavage. If you remove one of the four cells that was formed after the second cleavage in a snail, the embryo will be inviable because many of the necessary body organs will not form.