Cell lineage in the development of invertebrate nervous systems.

Studies of developmental cell lineage, i.e. of the fate of individual blastomeres that arise in an embryo, were begun in the 1 870s , in the context of the con­ troversy then raging about the "biogenetic," or "recapitulation," law promul­ gated by Ernst Haeckel . The biogenetic law seemed to imply that the early, pre-gastrula stages of metazoan embryogenesis recapitulate the nondifferen­ tiated condition of a remote colonial ancestor. Hence, prior to gastrulation, all blastomeres should be of equivalent developmental potency. Only after gastrulation would particular domains of the embryo become committed to the differentiated tissues characteristic of more recent metazoan ancestors. This implication was tested by a group of American biologists, led by Charles 0. Whitman ( 1878, 1 887). By observing the cleavage pattern of early leech embryos, Whitman traced the fate of individual blastomeres, from the uncleaved egg to the gastrular germ layer stage and concluded that, contrary to the simplest interpretation of the biogenetic law, even the earliest blastomeres are developmentally distinct and that each identified blastomere, and the clone of its descendant cells, plays a specific role in later development. Whitman set the pattern for all subsequent lineage studies. His disciples, including such future leaders of American cell biology as E . B. Wilson, E. G. Conklin, and F. R. Lillie, studied the embryos of other annelids, ascidians, and molluscs. Comparisons of their data revealed significant cross-phyletic similarities, as well as differences, in developmental cell lineage relations. Hence they concluded that there must be some relation between on�ogeny and