72 



THE SOCIAL LIFE OF ANIMALS 



up for the crowded eggs. Typical results and some 

 of the methods are shown in Figure 9. With appro- 



4- mm.' 



FifSt 



Second 

 % cleai/ed 



f^frst 

 Second 

 % cieaued. 



58.25 60.25 



85.83 90.25 



99 /GO 



Fig. 9. Eggs of the sea-urchin, Arhacia, cleave more 

 rapidly in dense populations than if only a few are 

 present. Figures below the diagrams, unless otherwise 

 indicated, give time in minutes. 



priate experimental precautions, some eighteen hun- 

 dred eggs were introduced into a tiny drop of sea 

 water. Near by on the same slide forty similar eggs 

 were placed in a similar drop and the two were 

 connected by a narrow strait as shown in the figure. 



