Because though the mice commonly used in laboratories are structurally more similar to human beings, fertilization and gestation take place inside the body in mammals. At birth, the heart and eyes are already fully developed, so that there is no opportunity to directly observe the development process.
因為實驗室常用的老鼠雖然構造上和人比較接近，但哺乳動物是體內受精、體內發育，一出生心臟和眼睛就已經發育完成，無從觀察起。