AbstractThe chick embryo is a convenient experimental system for embryologic and teratologic studies. However, windowing eggs during the first day of incubation, a procedure that is required to expose embryos in ovo, frequently results in dysmorphogenesis. The frequency and severity of the particular anomalies caused by windowing are greater the younger the embryo at the time this procedure is done. At all stages examined, dysraphic defects of the neural tube are the most common anomaly present following windowing. Defects of the neural tube are virtually eliminated if the air space introduced over the embryo by windowing is filled with albumen or saline, and the egg (with its window sealed with tape) rotated 180°, so that the embryo rests subjacent to an undisturbed area of the shell. Subblastodermic injection of saline, a vehicle often used for teratogenic agents, has no adverse effects when eggs are subsequently filled with albumen or saline and rotated. Furthermore, known teratogens (e.g., colchicine, hyaluronidase) injected subblastodermically after windowing are active in eggs that are then filled with albumen or saline and rotated. Finally, the addition of albumen or saline, followed by rotation, may be delayed up to 3 hours without reducing the restorative effects of these two procedures. Our modified procedures for the handling of windowed eggs significantly increase the value of young chick embryos for studies of early developmental events