Preformation and Epigenesis in the Origins of the Nervous System and Behavior: Issues, Concepts, and their History

Abstract

One of the most enduring and fundamental debates in the history of biology was over the problem of how in each generation a complex, functioning organism could arise from the vastly more simple fertilized egg derived from the parents. This ancient inquiry into the process of individual development marked the beginning of an interest in ontogenetic issues. Although philosophers and naturalists as far back as Aristotle were concerned with this question, it was not until the 17th and 18th centuries that the problem of development became a matter of sustained and serious interest.

Initially, the two major explanations of the process of development, preformation and epigenesis, appeared to be so different as to be virtually irreconcilable. Preformation theory argued that the fertilized egg, although apparently formless and unorganized, contained a full set of adult organs and characters that were too small to be seen. Thus, what appeared to be the de novo formation of tissues, organs, and other structures was, in fact, considered to be merely an increase in size. By contrast, proponents of epigenesis, beginning with Aristotle, held that the fertilized egg was, in fact, formless and unorganized and that ontogeny was actually a process whereby organs arose de novo by a gradual differentiation of the unorganized egg into an adult organism. Since virtually all of the pertinent facts of cell biology and embryology were unknown prior to 1700, both camps were forced to take refuge in metaphysics and theology for a “full” explanation of ontogeny.

Gradually, however, as more and more information became available increasingly sophisticated theories replaced the older views, although most biologists continued to align themselves with one or the other camp until late in the 19th century. It was only when the understanding of cell biology became sufficient to provide a rationale for experimentation that the problem of development was finally resolved. Thus, by 1900 a sketchy but, nonetheless, valid outline of the developmental process was available that has served as the foundation for all subsequent progress in this field.

Not surprisingly, by the opening of the 20th century a number of psychologists and biologists had shown a growing interest in how this new information in embryology and cell biology could aid their understanding of neural and behavioral development. With a few notable exceptions, however, most of them failed to fully comprehend either the facts or the significance of the theoretical resolution of the preformation-epigenesis debate. Consequently, new, albeit familiar, debates arose over similar issues, which were now couched, however, in terms of nature vs. nurture, heredity vs. environment and maturation vs. experience.

Much of this chapter is devoted to tracing the history of these debates in the late 19th and early 20th century. One of my major contentions is that if biologists and psychologists interested in neurobehavioral development had made a more concerted and sincere effort to understand the details of the earlier preformation-epigenesis controversy, as well as the facts supporting its subsequent resolution, then the resulting long and often bitter arguments over the nature vs. nurture issue would have been more quickly recognized as anachronistic and counterproductive. Only recently, more than 80 years after the fact, do a majority of workers in this field finally appear prepared to acknowledge what was almost a commonplace to most leading biologists at the turn of the century, namely, that ontogeny is the result of a preorganized code in the genes that in concert with a long series of epigenetic events, involving intra- and extraorganismic stimuli, together, gradually transforms the fertilized egg into the complex, adult animal.

to the mind capable of curiosity and wonder, the embryo is the most seductive object in nature

attributed to T. H. Morgan