Abstract
Since the attention of investigators was first directed to a study of animal embryos, their similarity in form and external detail even in the case of embryos of widely divergent species (especially in their early stages) and the general conformity to a basic plan of the various organs and organ systems has been recognized. At first no particular significance was attached to these resemblances, the similarity being taken more or less for granted. About the turn of the 19th century some of the outstating biologists of the period, notable Kielmeyer, Oken, Meckel and Serres, for reasons not entirely clear, began to teach that some relationship existed between the embryonic forms of higher animals and adult forms of lower ones. Evidently because these embryonic forms were not very critically examined, it finally came to be taught that the early worm-like stage of an embryo was identical with the adult form of certain of the worms. By thus associating the embryonic stages of complex animals with adult stages of simpler ones, the groundwork was laid for the evolutionary implications of what cam to be know as the recapitulation theory.