In this lab we will look at some early stages of development in three deuterostomes, sea urchin, frog and chicken. See cladograms for pattern of relationship. Eggs differ in the amount and location of yolk. Yolk contains stored nutrients the organism will use in development. This is carried to an extreme in reptiles, birds and monotremes where the yolk is restricted to a membrane bound yolk sac.
Penetration is when the sperm and egg meet and the sperm makes it way into the egg cytoplasm.
The rest degenerate. In mammals, the first sperm penetration triggers a response that prevents further sperm entry. In mammals, activation also results in the second mitotic division. In other animals, meiosis is complete at this stage. Fertilization is complete when the sperm and egg nuclei fuse.