The Development of Two Tailless Mutants in the House Mouse.

HE events that take place in the development of the mammalian T embryo have not been subjected to an extensive causal analysis so far. The reasons for this are“to be found mainly in the lack of suitable methods. It is not possible yet to use transplantation, isolation or vital staining techniques on mammalian embryos as they have been used on amphibian embryos. In the course of time it probably will be possible to analyze the mammalian embryo by transplantation and isolation just as thoroughly as has been done with the amphibian. For the present, however, the experimenter is not able “to take an active part in the course o i events that take place during the embryogeny of the mammalian embryo,” nor “to alter the course of events a t a chosen point in a chosen manner and draw conclusions on their relations from the resulting changes.” (SPEMANN 1936.) A mutation that causes a certain malformation as the result of a developmental disturbance carries out an “experiment” in the embryo by interfering with the normal development a t a certain point. By studying the details of the disturbed development it may be possible to learn something about the results of the “experiment” carried out by the gene. However to discover anything about the nature of the action of the gene is a much harder task. It is necessary for this purpose to be able to trace back all the results of the action to certain original causes. While the experimental embryologist carries out a certain experiment and then studies its results, the developmental geneticist first has to study the course of the development (that is, the results of the developmental disturbance) and can then sometimes draw conclusions on the nature of the ‘fexperiment” carried out by the gene. In amphibian embryology the experimental analysis has led to an understanding of the causal relations of the normal developmental events. It is possible that the study of hereditary developmental disturbances and their causes in the mammalian embryo may contribute to knowledge of the causal morphology of mammalian embryology. This paper restricts itself to the description of the development of an ’ This research was aided by a grant from the Josiah Macy Jr. Foundation.