LARVAE OF ARBACIA PUNCTULATA 353 



produced directly upon the developing embryo during the 

 periods of blastula and gastrula formation. 



Vernon ('95), working upon Strongylocentrotus, concluded 

 that subjection for one minute to a temperature above or be- 

 low that of the control was quite as effective in producing an 

 ultimate reduction in size as exposure for 1 hour. In his ex- 

 periments with sea-water of increased and decreased densities 

 ('00), he found that a brief subjection to diluted sea- water 

 (for example, from the time of fertilization until 6 or 12 hours 

 after) produced a decrease in growth rate, whereas continued 

 exposure (1 to 144 hours) brought about an increase, and he 

 suggested that primary reduction in size was due to shock 

 attendant upon transfer from dilute to normal sea-water. No 

 such effects, he believed, resulted from transfer from normal 

 sea- water to dilute; for ova used in the same experiment, which 

 were kept for 5 and for 24 hours respectively in normal sea- 

 water and then transferred to dilute sea-water for the remainder 

 of the period, resulted in a 2.6 and 2.8 per cent increase of size. 

 But it seems probable that there may have been a 'shock' quite 

 as great in the latter instance as in the former. In the first 

 experiment, however, when he subjected the larvae for the 

 briefer periods to the dilute sea-water, they were removed 

 from the more favorable medium before the effects of the pri- 

 mary inhibition could be completely overcome. But when they 

 were left for a longer time (such as, 144 or 192 hours) in this 

 medium, the effects of the more dilute solution had sufficient 

 time in which to compensate for both transfers and an ulti- 

 mate increase in size w^as induced. The same explanation 

 could be offered for the second experiment, when the larvae 

 were kept for the first few hours in normal sea-water and then 

 transferred to a more dilute medium. The ultimate effect of 

 this later long subjection to more favorable conditions was, as 

 we should expect, a growth more than sufficient to compensate 

 for the shock produced by short exposure to the more concen- 

 trated medium. Had this subjection been of longer duration 

 and treatment with dilute medium briefer, he might have 

 obtained larvae of normal or even of reduced size. 



