308 LOEB. 



unfertilized eggs of these forms after having resided in " normal " 

 sea water for about twenty-four hours begin to cleave shortly 

 before death. This cleavage, however, never goes beyond the 

 two- or four-celled stage. This might be explained by the fact 

 that the eggs begin to die at this time. After I had found this 

 year that the eggs of sea-urchins can still be fertilized after a resi- 

 dence of five days in sterilized sea water (at summer tempera- 

 ture ) , I decided to study this question of spontaneous cleavage 

 somewhat more closely. If it were true that individual sea-urchin 

 eggs begin to cleave in ordinary sea water after about twenty 

 hours, and cease to develop any further only because they soon 

 die, it would be expected that many or all should cleave when kept 

 alive four or five days, and that a number of them should reach 

 a fairly advanced stage of development. A lot of sea-urchin eggs 

 were distributed into a series of flasks containing sterile sea water. 

 One of the flasks was opened every morning and a careful search 

 was made for developed eggs. 



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

 either in the two-celled stage or in later stages of development. 

 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 mem- 

 brane 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 disavow that a few small eggs were present, 

 which perhaps 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 experi- 

 ment shows that in sea-urchin eggs also the processes of matura- 

 tion are not continuous with those of cleavage, and that entirely 

 different conditions which we can bring about through the abstrac- 

 tion of water or the entrance of a spermatozoon are necessary that 

 division may occur. It cannot be urged that the sterilized water 

 perhaps prevented the cleavage. When at the conclusion of the 

 experiment these same eggs were fertilized in sterilized water by 

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



