Certain Laws of Variation. 9f> 



The next and last experiment was made in August, when the tem- 

 perature of the water was on an average 24'5. In this case the larvae 

 were first kept in pure sea water collected several kilometres from the 

 shore. The specific gravity of this water was 1 '02868 at 15-56, that 

 of the normal tank water being 1-02901. 



Normal larvas (24-5) lOO'OO 



1 4 hours in pure sea water ... 90*38 



18 ... 93-61 



1-12 ... 96-98 



1-22 ... 97-04 



1_H4 ... 103-61 



Here we see that the larvae were reduced to their minimum size \yy 

 only three hours' development in pure sea water, and that longer 

 treatment produced a more and more favourable effect, though only 

 those larvae kept for the whole period of development in the pure water 

 were larger than the normal. 



We see, therefore, that in each of these series of experiments, 

 though the ultimate effect of the diluted or pure sea water was a 

 favourable one, yet a temporary immersion in it was always unfavour- 

 able. The times of production of the maximum diminution of size 

 were respectively twenty-four, ten, and three hours, or apparently 

 very variable. It is to be noticed, however, that these times more or 

 less correspond with the period at which the developing ova reach the 

 free-swimming blastula stage. Thus at a temperature of 13-8 this 

 was found to be some twenty to twenty-four hours, whilst at a 

 temperature of 24 it was about five hours. At 21-5 it is probably 

 about eight hours, though no exact observations were made to deter- 

 mine it. 



To what is this unfavourable effect upon the larval growth due ? It 

 is impossible that the pure sea water can of itself be a less favourable 

 medium for the early stages of development than the impixre tank 

 water, and probably the same is true as regards the diluted tank 

 water. In all probability the harmful effect is to be attributed to the 

 shock attendant on the transference of the embryos from water of a 

 lesser degree of salinity to that of a greater. Owing to the 

 differences of osmotic pressure thereby set up, the tissues would 

 immediately undergo a certain amount of shrinkage, and it is a 

 ready assumption that their growth is thereby for a time delayed* 

 The sensitiveness of the embryos to a change of salinity would seem 

 to be less and less the more advanced the state of development, so that 

 after a day or two's growth the harmful influence becomes entirely in 

 abeyance. 



The reverse process of transference of the developing embryos from 

 more saline to less saline water does not, on the contrary, appear to be 



