ABSTRACT
Our understanding of the mechanisms involved in the development of fetal behavior has greatly benefited from the use of animal models. One of these, the chick embryo, provides an opportunity for extensive observation and experimental manipulation of the developing embryo without the technical complications introduced when dealing with the mother. Ultimately, of course, the interactions between fetus and mother must also be addressed, but the advantages of studying fetal behavior in isolation should not be overlooked. For this purpose, chick embryos are ideal because fertile eggs are available throughout the year and the embryos develop readily in an incubator with little care other than maintenance of temperature and humidity. In addition, each embryo is encased in a hard, protective shell, which provides a secure environment for the developing chick. Access to the embryo can be obtained by simply making a hole or "window" in the shell, through which the embryo can be observed or manipulated (Hamburger & Oppenheim, 1967) . As long as sterile conditions are maintained, the window in the egg can be covered with tape or a glass coverslip sealed with paraffin and the egg returned to the incubator so that development can continue (e.g., Hamburger, Wenger, & Oppenheim, 1966).
