470 TREADWELL. [Vol. XVII. 



as incomplete or regional. If cleavage had the differential 

 and determinate character urged by Wilson and Conklin, I do 

 not see why all differential cleavages in gasteropods and anne- 

 lids, or at least in two genera of annelids, should not show 

 complete cell homologies. The number of divisions after dif- 

 ferentiation might vary, depending on physiological needs of 

 the organism, but tip to the stage of complete differentiation I 

 do not see why, on this theory, any incompleteness in the 

 homology should appear. Conversely, if complete cell homolo- 

 gies do exist between the different groups, then the cleavage 

 must have a differential and mosaic character. The evidence 

 is, I believe, against both assumptions. 



If cleavages are differential and have a phylogenetic signifi- 

 cance, why should the mesoderm in Nereis not arise by the 

 same number of divisions as in Podarke, since the two genera 

 are pretty closely related } Why should Arenicola completely 

 separate its mesoderm with the formation of 4d, while Aricia 

 requires one more division, and Nereis three or four, to com- 

 plete the separation between mesoderm and entoderm .-* Exam- 

 ples of this sort might be multiplied, showing, I believe, very 

 grave difficulties in the way of cell homologies and determinate 

 cleavage. 



What explanation are we to give, then, for these remarkable 

 resemblances .'' Are we to regard them as mechanically pro- 

 duced, and dependent upon conditions of pressure, temperature, 

 etc. .'' Certainly not. I believe that these resemblances are 

 due to homologies having perhaps a phylogenetic significance, 

 but to homologies which are not bounded by definite cell areas. 

 I believe that the prototroch, the paratroch, the supraoeso- 

 phageal ganglia, the mesoderm, are to be regarded as homolo- 

 gous structures, and that the fact that they do not lie within 

 corresponding cell walls in the different cases is an indication 

 that we are here dealing with a regional as distinct from a cell 

 homology. 



Since the ancestral form made use of a certain cleavage pat- 

 tern, secondarily moulding morphogenetic processes on cleav- 

 ages which were primarily mechanical, we would naturally 

 expect its descendants to follow in general the same cleavage 



