422 GRACE MEDES 



which might be attained at the 120 hour period or later, or 

 might be passed as early as the 72. During the latter portion 

 of the summer, growth was much slower, especially the first 

 day, and continued steadily for the 120 hours that the larvae 

 were under observation. The ultimate size, however, was 

 not so great as during the early period. The averages of the 

 maximum sizes during the two seasons were 86.23 and 82.25 

 respectively. But these distinctive characteristics were not 

 absolute with the various periods, — for instance, in the second 

 part of the summer (Period C), one of the seven averages 

 reached its maximum at the 72 hour stage, and in all the periods 

 there was considerable variation as to size of skeleton at the 24. 



Usually those that underwent the most rapid growth during 

 the first day were the ones that reached an early maximum. 

 We have a typical instance of such an early maximum with high 

 initial rate of growth in Experiment 21, CVIV, one of the cul- 

 tures in which the eggs from different females were fertilized 

 with sperm from different males. But this again does not 

 hold absolutely, as in BVI, another culture of the same experi- 

 ment, the measurements on the first day were exceptionally 

 high, yet growth was continuous for the entire time. 



Since, then, modifications due to environmental factors 

 which could not be controlled under the conditions of these 

 experiments, and modifications referable to different seasons, 

 were insufficient to explain the wide variation that occurred 

 in the larvae of the control cultures, other causes must be 

 sought. Among the factors involved, as the above experiments 

 show, are: variations in the degree of alkalinity; variations in 

 the proportions of the different constituents of sea-water, which 

 may be associated with changes in density; and the presence of 

 the products of metabolism. 



No definite data for the comparison of the alkalinity of the 

 sea-water seem available. Determinations have been made in 

 various regions, but owing to a difference in terminology and 

 in the indicators employed, definite comparisons cannot be 

 drawn. Loeb states that the concentration of the free OH 

 ions in the sea- water at Pacific Grove "seems to lie between 10"^ 



