LARVAE OF ARBACIA PUNCTULATA 



389 



eral average for the second period of the summer of 1915 (Plot 

 1 C) but during the following two days it increased more rap- 

 idly and exceeded the total average for the first part of the 

 summer (Plot IB). In Experiment 16, the control showed the 

 retarded growth characteristic of most of the experiments made 

 later in the season. At the age of 24 hours, it was only 2.93 

 or 19.1 per cent of that of the total average of all the experi- 

 ments (Plot 1 D) and 33.3 per cent of that of the general average 

 for the later period of that season (Plot 1 C). Growth was 

 very rapid the following two days, and at the age of 72 hours 

 it reached the highest point in its curve. This maximum is 

 considerably lower than that in any of the other experiments 



Plot 15 



in which it has been attained precociously, and again demon- 

 strates that rate of development is not necessarily correlated 

 with size. 



Figures 103 and 104 represent cultures g and h at 72 hours 

 and show, together with the reduction in size obtained in hyper- 

 tonic solutions, the typical abnormaUties characteristic of NaCl 

 solutions: comparatively long and slender oral arms, and short 

 body-rods which at the posterior region are perforated by many 

 openings. In cultures i and j, the eggs underwent cleavage but 

 the larvae did not form skeletons. 



The following measurements were taken from a similar set of 

 cultures in which development was more gradual; the maximum 

 was not reached until the 120th hour. 



JOURNAL OF MORPHOLOGY, VOL. 30, NO. 1 



