Eggshell Caps: A Method of Rearing Chick Embryos Visible throughout Incubation

Abstract

To watch a living chick embryo develop is always interesting. To do this, we are usually content to follow the method of opening a number of eggs which have been incubated previously for the desired number of days, and to regard the exposed embryos as constituting a continuous series (see Miller and Blaydes).' However, many teachers of Biology have wished for a simple method of observing the same embryo day after day throughout its incubation. Such a method would impress the student with the continuous nature of development, which is a fundamental concept. It would focus attention upon the new features that develop day by day, thus impressing the student more strongly. Furthermore, it would reduce the number of eggs needed to demonstrate the entire course of their development and hatching. The authors2 have reported recently the successful incubation of chick embryos with the so-called eggshell cap method. It is so simple and easily followed that it should serve as a workable and interesting laboratory project in high school biology classes. In addition to the advantages listed above, it presents the growing chick still within its shell so that the normal interrelations between the organism and its environment are maintained. This method involves the removal of the eggshell and both eggshell membranes, to expose the embryo and its extra-embryonic membranes directly beneath. The embryo may thus be observed directly and is subject to manual manipulation. The method of preparation is simple: First, an eggshell cap is prepared by breaking an egg at the small end, emptying the contents and, with scissors, eutting the empty shell around its middle. The result is approximately a half eggshell with a cross-section similar to Fig. A. It is then dried and both shell membranes are removed, as in Fig. B. The eggshell cap taken from the large end of the egg consists then of the shell only. It is sterilized with alcohol, dried, and kept in a covered container until used. Eggs to be incubated are opened prior to ineubation. This point seems to be essential, to avoid injury to extraembryonic membranes and blood vessels which is so likely to occur once they are formed. Holding the egg vertically, with the large end uppermost near a bright lamp, a light area marks the limits of the air cell in that region After cleaning the eggshell with cotton moistened with alcohol, the outline of the air cell is marked with pencil. With tweezers a small opening is made above the air cell, 1 Miller, D. F., and G. W. Blaydes, Methods and Materials for Teaching Biological Sciences. N. Y.: McGraw-Hill Book Co., 1938, pages 402403. 2 Price, J. W., and E. V. Fowler. Science, 91: 271, 1940.