brates and invertebrates, aided by the then-recent improvements in light microscopy and in staining methods and stimulated by Darwin’s proposals that the study of ontogeny (i.e., the animal’s embryonic development) holds clues to phylogeny (i.e., its evolutionary origin). Among the highlights during the period of 1880-1940 were the detailed anatomical descriptions of developmental stages of embryos, including the first atlas of human embryos, reconstructed from microscopic sections, published by W. His, Sr., in 1880-1885. In vertebrate embryology, these descriptions revealed the organogenesis of the heart, kidney, limbs, central nervous system (CNS), and eyes. Developmental-fate mapping studies revealed the embryonic sites of the origin of cells of the organs and the rearrangements of groups of cells in morphogenesis. The stages of development were found to include, in reverse order, cytodifferentiation, organogenesis, morphogenesis (gastrulation and neurulation), rapid cleavage, fertilization, and gametogenesis. By the 1940s, anatomical descriptions of the embryos of related animals were integrated into coherent evolutionary schemes, taught in comparative embryology classes, revealing, for example, the modification of the gill slits of jawless fish to the jaw of jawed fish and further modification to the middle ear of mammals. Also, by this time, Haeckel’s oversimplified scheme had been abandoned, namely, that ontogeny merely recapitulates phylogeny.
Experimental embryology also began in the late 1800s. In experimental studies, which mostly involved techniques of cell and tissue transplantation and removal, the central role of cytoplasmic localizations and cell-lineage-restricted developmental fates was recognized in the development of certain invertebrates by the early 1900s. In vertebrate development, the importance of inductions (also called tissue interactions) was recognized in the 1920s, following the stunning organizer transplantation experiments by Spemann and Mangold (1924) on newt embryos. By the 1950s, inductions had been found in every stage and place in the vertebrate embryo, for example, in all the kinds of organogenesis. Vertebrate development, including that of mammals, had become comprehensible as a branching succession of inductive interactions among neighboring members of an increasingly large number of different cell groups of the embryo.
Developmental mechanisms, as understood even in the 1970s, were descriptions of the movements and interactions of cells or groups of cells. They were cellular- or tissue-level mechanisms. The all-important “inducers” were materials of unknown composition released by one cell group and received by another group. Consequently, the recipient cells took a path of development different from the one that would have been taken if they were unexposed. The progression or momentum of development also was recognized: that the individual events of interactions and responses are time-critical, and that certain subsequent aspects of development never occur if one event is prevented.
Molecular mechanisms, however, were not understood at that time. Embryologists encountered the limits of the field in the 1940-1970 period, as they tried to discover the chemical nature of inducers and the responses of cells to them.