RHYTHMICAL SUSCEPTIBILITY OF SEA URCHIN EGGS. 



255 



of each lot was removed from the latter solution after 173^, 

 20, 25, 30, 35, and 40 minutes, and allowed to develop in normal 

 sea water. The percentages given in the table show the degree 

 of blastula development in each culture. Repetitions of the 

 experiment showed no significant variation in the optimum 

 exposure to the hypertonic solution with changes in the time 

 the eggs remained in the normal sea water after acid treatment. 



Table I. 



T° = 20° - 22°. 



In order to determine if the eggs of Arbacia punctulata which 

 had been treated with acidulated sea water, showed a rhythmicity 

 in sensitiveness to the hypertonic solution, such as Herlant's 

 experiments with the eggs of Paracentrotus lividus indicated, 

 the following experiments were carried out. The eggs of several 

 sea urchins were collected, treated with sea water made acid by 

 the addition of 2 c.cm. N/io butyric acid to 50 c.cm. of sea water. 

 After remaining in this solution for from 2 to 23^2 minutes the 

 eggs were transferred to normal sea water. At the end of 5- 

 minute intervals lots were removed to finger bowls containing 

 hypertonic sea water [50 c.cm. sea water + 8 c.cm. 23^2 M 

 (NaCl + CaCl2 + KCl)]. After remaining in the hypertonic 

 sea water for 25 minutes the eggs were put into normal sea water 

 and allowed to develop. The percentages of advanced morulse 

 or non-swimming blastulse were determined by counting random 

 fields. The following table (Table II.) gives a typical result, 

 showing optimum effects when the eggs were put into the hyper- 

 tonic solution 40, 60, 90-100 and 115-125 minutes after acid 

 treatment. The rhythmical character of the result is obvious 

 from Curve I., where the ordinates indicate the percentage of 

 larvae formed, while the abscissae indicate the time which elapsed 

 between acid treatment of the eggs and their exposure to hyper- 

 tonic sea water. 



