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This little volume is the sixth of the very excellent science series being published by The University of Chicago Press, which aims to take up specific aspects of scientific problems and discuss them in a way not too technical for the average reader.
The Biology of Twins is a careful study more particularly of the embryological characteristics of the multiple births of the nine-banded armadillo. This animal gives birth, almost without exception, to four young as the result of each pregnancy, and as they exist in large numbers are readily available for study. The principles of twinning which are worked out can be given wide application to all mammals. The principal differentiation of twins is as to whether they are fraternal, that is, dizygotic, or duplicate, that is, monozygotic. Human twins may belong to either class, and a study of the armadillo shows that the previous standard for determination, namely, whether there be one or two chorions, is fallacious, inasmuch as twins from two separate eggs may be so crowded in the uterus that the chorions fuse and make it appear that they are of monozygotic origin.
There is an exceedingly interesting chapter on variation and heredity and the application of the Mendelian law to the study of the number and the doubling of the scutes in the nine bands. This particular chapter winds up with a discussion of some of the principles of bilaterality, particularly with reference to mirror-image types, this latter illustrated by finger prints of the friction-skin patterns.
The reason why the skin pattern should show mirror-image bilaterality and there should be no corresponding bilaterality with reference to the internal organs is because the ectoderm is dominant and it is in the ectoderm that the twinning fission begins.