24 BIOLOGICAL LECTURES. 



cases by every criterion at our command. A long series of later 

 researches, beginning with Whitman's epoch-making studies 

 on the cleavage of Clepsinc, has demonstrated analogous facts 

 in the case of many other cells of the cleaving ovum, and has 

 finally shown that in many groups of animals (though appar- 

 ently not in all) the origin of the adult organs may be determined 

 cell by cell in the cleavage stages ; that the cell-lineage thus 

 determined is not the vague and variable process it was once 

 supposed to be, but is in many cases as definitely ordered a 

 process as any other series of events in the ontogeny ; and that 

 it may accurately be compared with the cell-lineage of other 

 groups with a view to the determination of relationships. 



The study of cell-lineage has thus given us what is practi- 

 cally a new method of embryological research. The value and. 

 limitations of this method are, however, still under discussion, 

 and among special workers in this field opinion as to its mor- 

 phological value is still so widely divided that most of its results 

 should be taken as suggestive rather than demonstrative. Like 

 other embryological methods, it has already encountered con- 

 tradictions and difficulties so serious as to show that it is no 

 open sesame. In some cases closely related forms {e.g., gastero- 

 pods and cephalopods) have been shown to differ very widely, 

 apparently irreconcilably, in cell-lineage. In other cases (echi- 

 noderms, annelids) the normal form of cleavage has been 

 artificially changed without altering the outcome of the devel- 

 opment. In still other cases {e.g., in teleost fishes) the form 

 of cleavage has been shown to be variable in many of its most 

 conspicuous features, so that apparently no definite cell-lineage 

 exists. These and many other facts, less striking but no less 

 puzzling, can be built into a strong case against the cell-lineage 

 program, and I wish to acknowledge its full force. Admitting 

 all the difficulties, I am nevertheless on the side of those who as 

 morphologists believe that the study of cell-lineage has demon- 

 strated its value, and that it promises to yield more valuable 

 results in the future. In this lecture I propose to illustrate 

 some of the more interesting results already attained, and some 

 of the suggestions that they give for future work, by a broad 

 consideration of the cell-lineage of three related groups of 



