26 WILSOX. 



ingenuity by Roux and Weismann, the pendulum of opinion 

 may not have swung too far towards the opposite extreme. 

 The persistence in cleavage of vestigial cells (such as the rudi- 

 mentary enteroblasts of Aricia), or of vestigial processes in the 

 formation of the germ-layers (as in the origin of the " mesen- 

 chyme ' ' in i^/iio or Crepididd) adds to the evidence that the 

 number and character of the cell-divisions stand in some direct 

 and important relation to the differentiation -process ; and it 

 would be difficult to explain such ancestral reminiscence in cell- 

 lineage under any view w'hich does not recognize in cell-out- 

 lines the definite boundaries of differentiation-areas in the de- 

 veloping embryo.^ The history of the posterior cell of the 

 fourth quartet in annelids and gasteropods gives a clue to the 

 process through w^hich teloblasts and other determinate proto- 

 blasts have arisen by progressive specialization ; and I think it 

 lends support to the distinction drawn by Conklin^ between 

 " determinate " and ''indeterminate " types of cleavage by show- 

 ing some of the steps by which the former may ha\'e been 

 acquired. 



From a physiological standpoint the persistence of rudimen- 

 tary cells in cleavage is a problem of high interest which 

 merges into the larger problem of ancestral reminiscence in 

 general. When one considers the analogous case of the polar 

 bodies, one is almost tempted to suspect that the formation of 

 the rudimentary enteroblasts may be in some wa\' connected 

 Avith a definite transformation of the nuclear substance. It is, 

 however, equally possible that the removal of the cytoplastinic 

 substance of these cells may be a necessary condition of the 

 differentiation of the mesoblastic material. 



Zoological Laboratory of Columbia University, 

 December 4, 1897. 



1 Cf. Wilson, 1893, P- ^4- 



2 1897, p. 190. 



