26o LILLIE. [Vol. XVII. 



Child ('97) finds a difference in the size of the cleavage 

 asters, and suggests that " this anticipates and indicates the 

 difference in size of the first two cells of cleavage." The 

 segmentation nucleus is shown in an excentric position in the 

 figure illustrating this, and I would suggest that the difference 

 in the asters may be due to this. 



2. Conklin ('99) ascribes tmequal cleavage to 7novcniC7its of 

 the cytoplasm : " When the cell movements carry the mitotic 

 spindle out of the middle of the cell, unequal cleavage results." 

 The nature and cause of the movements are thus described : 

 " With the escape of nuclear sap into the cell body at the 

 beginning of mitosis, vortical movements are set up in the 

 cytoplasm, the poles of the spindles being the centers of such 

 vortices." The movements are thus " due to the appearance 

 of unlike substances in different parts of the cell." The 

 extremely delicate and beautiful observations on which Conk- 

 lin's conclusions rest form a model of morphological methods. 

 I fully believe that we have here a factor that must be reckoned 

 with in our theories of early differentiation ; it may even have 

 all the importance that Conklin claims for it, while it certainly 

 fails, like all its predecessors, to give any clue to the solution 

 of the adaptive aspect of these phenomena. It may well be 

 that the shiftings of the first cleavage spindle in Unto are due to 

 currents in the cytoplasm ; the changes in form of the sphere 

 substance and the elongation of the cells after cleavage furnish 

 possible evidence of such currents. But why these currents 

 should relate themselves to subsequent events by placing the 

 spindle in its definite position we cannot say. It would seem 

 also to be necessary to distinguish carefully between random 

 cytoplasmic streamings and those that are definitely adapted 

 to purposes of differentiation. For instance, Ziegler ('96) de- 

 scribes how the germ-nuclei in the egg of Diplogaster, lying 

 near opposite ends of the egg, are carried around at first in an 

 aimless fashion by streaming movements of the cytoplasm, 

 which do not, however, prevent the union of the germ-nuclei 

 near the end of the egg, destined to become the posterior end 

 of the embryo. The spindle is then formed and rotates so as 

 to lie in the long axis of the egg, frequently oscillating for a 



