286 H. J. VAN CLEAVE 



The difference in the number of cells offers no difficulty in the doctrine 

 of cell homology unless we assume that all divisions are differential, a 

 thing which we know is not true. After blocking out the protoblasts of 

 various regions and organs an indefinite number of non-differential divi- 

 sions may occur either before or after the complete differentiation of the 

 parts, and this probably explains the larger number of cells in the em- 

 bryo of Crepidula adunca and the smaller number in the adult. In fact 

 after the complete differentiation of all the tissues and organs, the num- 

 ber of cells may vary greatly in the different individuals of the same 

 species or in the same individual at different times. In adult Crepidulas 

 the number of cells varies directly as the body size varies, the cell size 

 remaining practically constant. These later divisions, in the main, are 

 non-differential, and likewise it is probable that in the later stages of 

 cleavage many non-differential and inconstant divisions occur. Not 

 only is there greater variation in the number and size of cells in later as 

 compared with earlier stages of cleavage, but there is also greater vari- 

 ation in the direction and time of division; all of which goes to prove 

 that the earlier cleavages are more constant, more frequently differen- 

 tial, and therefore morphologically more important. 



In citing these instances wherein among the Protozoa and in 

 the embryology of the Metazoa constancy seems to have a be- 

 ginning I do not wish to give the impression that from such iso- 

 lated and fragmentary citations I would claim to have traced a 

 probable actual rise of fixity in numbers of cells going to make 

 up an adult individual. In fact I do not consider cell constancy 

 as of simple origin, for, while it occurs in broadly separated groups 

 of the animal kingdom, it has probably arisen independently in 

 each from the primitive, more variable conditions through the 

 processes which tend toward the elimination of variability dur- 

 ing the phylogeny of the group. 



3. Factors involved in the production of cell constancy 



For the explanation of the causes and limitations of cleavage 

 various factors have been suggested. Reference has already been 

 made to the works of Morgan as indicating that the production 

 of the anlagen of organs in individuals developed from isolated 

 blastomeres tends to require the same number of cells as required 

 in normal development, even though the abnormal individuals 

 are smaller than the normal and have fewer cells. Conklin's work 

 has also been cited, wherein he considers the earlier cleavages 



