Experiments conducted at the Hopkins Marine Station, Pacific Grove, California, during the summer of 1938 on the unfertilized and fertilized eggs of Dendraster excentricus have given certain interesting modifications in the early developmental patterns of this form.Unfertilized eggs treated for 12 hours with a solution of NaCNS (20 cc .54 M NaCNS plus 100 cc Ca-free sea water) showed a retardation in cleavage rate when returned to normal sea water and fertilized. The developing larvae ranged from forms showing considerable inhibition (increased mesenchyme, shortened anal arms, undifferentiated oral lobe) to larvae (approximately 35%) which consisted entirely of ectoderm and ectodermal structures. These ectodermized forms were modifications in the same direction as the ‘animalized’ forms produced in the sea urchin by Lindahl1 and Horstadius2 with NaCNS treatment and experimental isolation of animal blastomeres. Neither mesenchyme, pigment nor gut was present. With higher concentrations or exposure periods ...