452 CHARLOTTE HAYWOOD. 



not merely the mean time for all of the eggs hut, approximately 

 at least, the time for each individual egg. It is believed that data 

 of this sort, while more troublesome to secure, are theoretically 

 more significant and have a wider range of usefulness than those 

 obtained by the other method. 



METHOD. 



Sea water, saturated with CO 2 from a Kipp generator, was used 

 directly or was diluted as desired with oxygen-saturated sea water, 

 with nitrogen-saturated sea water, or with ordinary sea water by 

 siphoning together appropriate amounts of these solutions. It 

 was then immediately siphoned into 75 cc. glass-stoppered bottles 

 containing a few drops of a concentrated suspension of the newly 

 fertilized eggs of Arbacia punctulata. As soon as the bottles were 

 completely filled, they were tightly stoppered, shaken briefly, and 

 placed in a bath of running sea water, which, with a few excep- 

 tions, varied in temperature not more than 0.5 C. for the dura- 

 tion of each experiment. For all the experiments during one 

 season the range of temperature was 18.4 C. to 22.4 C., with 

 a mean value of 20.3 C. In order to prevent a change in the 

 solubility of the dissolved gases, it was deemed important always 

 to have the CCX-containing so'utions at the temperature of the 

 water-bath. Care was also taken to begin all the exposures of 

 any one series as nearly simultaneously as possible, since a num- 

 ber of experiments not reported here seemed to indicate that sen- 

 sitiveness to CO 2 may vary prior to the appearance of the first 

 cleavage. In many of the experiments the bottles were inverted 

 at five minute intervals to keep the contents well mixed, but other 

 experiments in which this precaution was less rigorously observed 

 gave essentially the same results. 



At the time of setting up an experiment, samples of the solu- 

 tions used were taken for estimations of the pH and dissolved 

 oxygen. The former were immediately determined with the use 

 of phenol red, brom thymol blue, brom cresol purple, and methyl 

 red as indicators ; the samples for the latter were kept at a constant 

 temperature until Winkler determinations could be made. The 

 results of the Winkler determinations are expressed as cc. of 



