76 MARY GRACE SPRINGER. 



that specified stage. The cell walls of the developing eggs pre- 

 sumably must be permeable to the formative stuffs present in the 

 solution, if any effect is to be registered. 



The whole of the work was done at the Marine Biological 

 Laboratory at Wood's Hole during the summer of 1921. The 

 writer is indebted to Professor C. G. Rogers for suggesting the 

 problem and for his criticism of the plan of work. 



METHODS AND MATERIALS. 



The animals used in these experiments were the starfish, As- 

 terias forbcsii, and the sea-urchin, Arbacia punctulata. The eggs 

 and sperm of both were secured by shaking the ovaries or testes 

 into a dish of sea water. The starfish eggs were filtered through 

 silk bolting cloth to remove portions of the ovarian tissue. The 

 animals were first washed under a stream of fresh water and all 

 precautions were taken to cleanse thoroughly the glassware and 

 instruments used. 



By " egg extract " is meant a suspension of crushed eggs in 

 sea water or other specified solvent. The eggs were centrifuged, 

 the fluid decanted, and the eggs ground up with fine sea sand, free 

 from impurities, and thoroughly washed by the solvent used. 



In some experiments the eggs were crushed between glass 

 plates, but this method was not satisfactory. No uncrushed 

 eggs were present in the solutions to invalidate the percentages. 

 Extracts of larvse were made up in the same manner. To main- 

 tain as uniform concentration as possible, a definite proportion of 

 eggs to solvent was kept (i c.c. eggs to 5 c.c. solvent) but the 

 concentration was probably lowered by the retention of some of 

 the protoplasmic fragments in the sand. 



The term " blastula w^ater " or " gastrula water," for example, 

 is understood to mean water in which these larvse have stood up 

 to that stage in their development. Both the eggs and sperm in 

 any one experiment were derived from one individual each when- 

 ever possible. The sperm suspension used w : as concentrated 

 enough to cause 100 per cent, fertilization. 



Data of the exact time of the 2-, 4- and 8-cell cleavages chiefly 

 are noted. Although it is difficult to say just when, in terms of 



