“The relation between normal and abnormal development of the frog's egg”

The method of development of the frog's egg may be changed by a number of external agents. If the eggs are revolved at the rate of 180 revolutions per minute; if they are put into salt solutions of definite strengths; if they are subjected to a low or to a high temperature; if they are deprived of sufficient oxygen or surrounded by carbon dioxid in solution; if they are placed on wet filter paper instead of developing under water—in any of these ways abnormal embryos result.

An examination of the effects of these external agents brings out two points of especial interest. First, that the effects are not gradual, i. e., corresponding in degree to the increasing strength of the agent employed, but that no effects appear up to a certain point and then suddenly the agent begins to act. Increasing the strength of the agent above this point may for a small range increase the effect, but this occurs within extraordinarily narrow limits compared with the lower range of non-action. The most plausible explanation of this mode of behavior in most of the cases is as follows: The agents act by coagulating certain parts of the egg, thereby preventing their further development. Other parts of the eggs that are made up of different colloids or of different concentrations of colloids remain unaffected, and proceed to carry out their development as far as the presence of the injured region allows.

The second point was the one that the author spoke of especially. Despite the great diversity in the form of the abnormal embryos, most of them may be reduced to modifications of the same type. For example, in many cases the dark cells of the upper hemisphere do not grow down over the lower hemisphere to produce there the embryo, but, remaining at the top of the egg, partially constrict off from the yolk cells at, or even above, the equator of the egg. Out of these dark cells the abnormal embryo develops usually in the form of a ring. Sometimes one side only of the ring develops and a half embryo appears; sometimes only the anterior end of the ring develops and an anterior embryo appears (often more or less “ open ”), etc.