THIRD LECTURE. 



ADAPTATION IN CLEAVAGE. 



FRANK R. LILLIE. 



I. INTRODUCTION. 



IT has happened, very naturally, that writers on the subject 

 of cell-lineage have laid special emphasis on the resemblances, 

 which are nothing short of marvellous from the older points 

 of view, between the cleavage of the eggs of even widely 

 separated forms. Over and over again it has been demon- 

 strated that in gasteropods, lamellibranchs, annelids, and tur- 

 bellarians, the ectoderm is made up of three quartets of cells, 

 formed from the first four cells ; that the fourth product of the 

 left posterior macromere contains the mesoblast, excepting in 

 the turbellarians ; that the turbellarian mesoblast is represented 

 in the other groups ; that the four basal cells, after the separa- 

 tion of the ectoderm and mesoblast, form endoderm ; that the 

 ectoderm of the entire trunk is derived from d 2 (speaking 

 technically), and that there is a wide-reaching sameness in the 

 cell-lineage of the prototroch, cross, and of other larval organs. 

 Ancestral reminiscences even have been discovered in the 

 cleavage (E. B. Wilson, 14). I do not in the least underesti- 

 mate the immense value of these results, which form one of 

 the most brilliant and interesting chapters in modern embryol- 

 ogy. But the tendency to schematize has naturally arisen, and 

 one of the most instructive aspects of cell-lineage is thus lost 

 sight of, namely, the special features of the cleavage in each 

 species, which are, I believe, as definitely adapted to the needs of 

 the future larva as is the latter to the actual conditions of its 

 environment. 



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