SECOND LECTURE. 



CELL-LINEAGE AND ANCESTRAL 

 REMINISCENCE. 1 



EDMUND B. WILSON. 



EVERY living being, at every period of its existence, presents 

 us with a double problem. First, it is a complicated piece of 

 mechanism, which so operates as to maintain, actively or pas- 

 sively, a moving equilibrium between its own parts and with its 

 environment. It thus exhibits an adaptation of means to ends, 

 to determine the nature of which, as it now exists, is the first 

 task of the biologist. But, in the second place, the particular 

 character of this adaptation cannot be explained by reference to 

 existing conditions alone, since the organism is a product of the 

 past as well as of the present, and its existing characteristics 

 give in some manner a record of its past history. Our second 

 task in the investigation of any problem of morphology or phys- 

 iology must accordingly be to look into the historical back- 

 ground of the phenomena ; and in the course of this inquiry we 

 must make the attempt, by means of comparisons with related 



1 This lecture is based on a paper entitled "Considerations on Cell-Lineage and 

 Ancestral Reminiscence, Based on a Reexamination of Some Points in the Early 

 Development of Annelids and Polyclades," in Ann. N. Y. Acad. Set., 1898. 

 In some passages the wording of that paper has been reproduced with only 

 slight change. With the exception of Fig. 4, the figures are entirely schematic and 

 are designed to show only the broadest and most essential topographical features. 

 For this purpose the subdivisions of the micromeres have been omitted, and, 

 except in Fig. 4, none of the figures represent the actual condition of the embryo at 

 any given period. While, therefore, very misleading in matters of detail, they are, 

 I think, true to the essential phenomena ; and through the simplification thus 

 effected the reader is spared a mass of confusing descriptive detail in no way essen- 

 tial to the broad relation on which it is desired to focus the attention. 



