78 BIOLOGICAL LECTURES. 



unsegmented egg. The constancy of cleavage in later stages 

 is associated with constancy of movements in the cytoplasm, 

 and it is probable that the same is true of the constancy which 

 precedes cleavage. 



That the movements within the cell substance of the unseg- 

 mented egg are, in certain cases at least, of a vortical character 



FIG. 2. 



FIGS, i AND 2. Sections of eggs of Crepidula plana, showing prophase and metaphase 



of first cleavage. 



is indicated by spiral asters, first described by Mark for Limax, 

 and since observed by several investigators in other animals, 

 and also by my observation that the first cleavage in Crepi- 

 dula is a spiral one, being oblique to the right, or dexiotropic. 



