24 IVILSOX. 



III. 



On Cell-lineage and Ancestral Reminiscence.^ 



The phenomena shown in the history of the micromere-quar- 

 tets in platodes, anneHds and moUusks are, I think, of general in- 

 terest in two directions. 



In the first place they render it highly probable, if they do not 

 actually demonstrate, that development may exhibit ancestral 

 reminiscence as clearly in the cleavage of the ovum as in the 

 later formation of tissues and organs. That the rudimentary 

 entoblasts of Aricia, Spio, or AmpJiityite are such ancestral rem- 

 iniscences seems almost as clear as that the yolk-sac of the 

 mammalian embryo or the primitive streak of a bird-embryo are 

 such ; and the same may be said of the formation of mesen- 

 chyme-cells from the second quartet in Uiiio or Crcpidiila 

 These facts, among many others, may well give us hope that, 

 when the comparative study of cell -lineage has been carried 

 further, the study of the cleavage-stages may prove as valuable 

 a means for the investigation of homologies and of animal rela- 

 tionships as that of the embryonic and larval stages. The re- 

 sults of experimental embryology have no doubt seemed ad- 

 verse to such a conclusion, by showing how easily the cleav- 

 age-stages may be altered by changes in the conditions of 

 development. But I cannot see that the embryonic and larval 

 stages are in much better case. Certainly the modification of 

 cleavage-forms which Driesch has effected in the echinoderm 

 ^S& by pressure, temperature and the like, are hardly greater 

 than those which Herbst has brought to pass in the gas- 

 trular and larval stages of the same eggs through modification 

 of the chemical environment. It is true that nearly related 

 forms — for example the gasteropods and the cephalopods — may 

 differ very widely in the form of cleavage ; but so they may in 

 the embryonic and larval stages, and it may fairly be questioned 

 whether ** secondary modification " or *' coenogenetic change " 

 has gone further in one case than in the other. 



^ The term "ancestral reminiscence" is here used to denote any feature of de- 

 velopment, the meaning of which is only apparent in the light of earlier historical 

 conditions, whether of the adult or of the embryo. 



