As one of the most simply structured parts of the central nervous system and because it is located accessibly outside the skull, the retina is often used in experiments. Since studies on humans, however, are generally not possible or practical, other animals are frequently exploited. The chick embryo is often a prime choice because it is a higher vertebrate that can easily be manipulated and observed during the entire process of embryonic growth through the use of eggs with a window in the shell or even eggshell free cultures.