The Development of Asymmetry

Abstract

It has been shown that the median plane of symmetry of the embryo, when not impressed on the egg by its stiff enveloping membrane, may in certain cases be traced back to another external agent, such as the path of entrance of the spermatozoon, or possibly, in one case, to pressure acting on the egg in the oviduct. It was not found necessary, therefore, to assume that a bilateral structure is present in the protoplasm of the unfertilized egg, as long as its origin could be explained as arising from extraneous sources. There is another group of cases in which structures that are asymmetrical develop. It is as important to discover the origin of asymmetry as to find out how a bilateral symmetry arises in the egg, and the situation is all the more interesting because the asymmetry is generally imposed on forms that have a fundamental, structural bilaterality.

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