744 STUDIES IN G-ENEKAL PHYSIOLOGY 



In the course of five days I never found a single divided 

 egg, either in the two-celled stage or in later stages of devel- 

 opment. It is possible that during the last days of the 

 experiment a few eggs divided, and that the cleavage cells 

 fell apart. Lewis and I found last year that when eggs 

 are fertilized forty-eight or more hours after their removal 

 from the ovaries they form no membrane and the cleavage 

 cells fall apart. I have corroborated this fact this year. 

 Usually more than one embryo develops from such an egg, 

 because the cells drop apart. I kept this fact in mind and 

 will not deny that a few small eggs were present, which per- 

 haps represented only half the mass of an ordinary egg. 

 But nearly all the eggs were of normal size, and since small 

 eggs are occasionally found even under normal conditions, 

 the experiment shows that in sea-urchin eggs also the 

 processes of maturation are not continuous with those of 

 cleavage, and that entirely different conditions which we can 

 bring about through the abstraction of water or the entrance 

 of a spermatozoon are necessary that division may occur. 

 It cannot be urged that the sterilized water perhaps pre- 

 vented the cleavage. When at the conclusion of the experi- 

 ment these same eggs were fertilized in sterilized water by 

 adding a drop of sperm, they developed to the pluteus stage 

 in sterile sea-water. I, therefore, consider it possible that 

 where authors describe a cleavage of the unfertilized sea- 

 urchin eggs in "normal" sea-water, the sea- water or the 

 egg had in reality suffered some change which had escaped 

 the notice of the observers. One might think of evaporation 

 and increase in the osmotic pressure of the sea-water. A 

 very slight increase in the osmotic pressure of the sea-water 

 is sufficient to cause the sea-urchin egg to divide into two 

 cells in the course of twenty hours. One might also think 

 of a change in the sea-water brought about by the putre- 

 faction of the dead eggs. Finally it is possible that a sub- 



