A Recapitulation of the Rise and Fall of the Cell Lineage Research Program: The Evolutionary-Developmental Relationship of Cleavage to Homology, Body Plans and Life History

Abstract

American biologists in the late nineteenthcentury pioneered the descriptive-comparativestudy of all cell divisions from zygote togastrulation – the cell lineage. Data fromcell lineages were crucial to evolutionary anddevelopmental questions of the day. One of themain questions was the ultimate causation ofdevelopmental patterns – historical ormechanical. E. B. Wilson's groundbreakinglineage work on the polychaete worm Nereis in 1892 set the stage for (1) an attackon Haeckel's phylogenetic-historical notion ofrecapitulation and (2) support for mechanisticexplanations of cleavage patterns. As morelineage work – especially Lillie's work on Unio and Conklin's on Crepidula – becameavailable in the mid-late 1890s, mechanism wastempered with more evolutionary, homology-basedviews. However, as I show by focusing on threemajor issues – homology, body plans and lifehistory – these views were primarily based onthe precocious segregation and prospectivesignificance – what the cell became not what itwas. Even on issues like adaptation, mostlineagists argued teleologically from the adultbackward. Most cell lineage workers, by 1900,were to varying degreesmechanist/experimentalist and recapitulationistsimultaneously. The exception was E. G.Conklin, whose views were more akin to aDarwinian evolutionist than either mechanist orrecapitulationist. Lineage work eventuallydeclined and by 1907 published accounts of newlineages had basically stopped. I argue thatestablished workers and younger researchersstopped wanting to take on cell lineageprojects because the general patterns were thesame for all the spiralians while the specificsshowed too much variation. It was hard totheoretically encompass or analyze the minutiaeof variation in a recapitulationist ormechanist framework. The only establishedworker who continued to do comparative lineagestudies was E. G. Conklin, perhaps because thevariation could best be accommodated byDarwinian evolution.