OCCURRING IN THE NEIGHBOURHOOD OF PLYMOUTH. 27 



temperature between the sea and the observed sample of sea-water 

 was that between 11'6° C. and 15-0° C, or 3'4°. Thus the density 

 of the water at the surface of the sea was slightly greater than that 

 observed in the samples^ allowing for the expansion of the water 

 caused by a rise of temperature of two or three degrees. But this 

 difference of density would be scarcely appreciable, and we may there- 

 fore conclude with certainty that the density of the surface water of 

 the sea was not less than 1'0267 and very little higher than r0269. 

 The temperature on June 12th, seven miles south-west of the Eddy- 

 stone^ was 11 "6° C, and on July lOth less than 13'3° C. 



The following data of the conditions of temperature and density 

 to which the ova were exposed during the Laboratory observations 

 are to be compared with the natural conditions ascertained above. 

 The first lot of ova I got, namely, those taken on May 24th, I placed 

 in a glass jar of water provided with a slow circulation ; at the 

 bottom of the jar was a layer of gravel, and a glass cylinder with 

 its base in the gravel surrounded the siphon throug-h which the 

 outflow took place. Thus the ova were prevented from approaching 

 the exit siphon^ while the water passed through the gravel up into 

 the inside of the cylinder. This apparatus was arranged at 10 a.m. 

 on May 24th, and on the following day I found that all the ova had 

 sunk to the bottom of the jar, and were in a dead or dying condition. 

 A few of the ova which had been left in the water brought in from 

 the sea were still alive and developing. The water supplied to the 

 circulation apparatus was brought up from the shore, as at this 

 time I was restricted to my single room in the Laboratory, and the 

 aquarium apparatus was not built. The result showed that the 

 water from the shore, although constantly supplied in a pure condi- 

 tion to the eggs in the circulation apparatus, was rapidly fatal, while 

 in the water from the open sea^ although perfectly still and un- 

 changed, the ova lived. 



On May 26th more ova were sent up by the crew of the " Prima 

 Donna.''^ I had some clean water brought up from the shore, and 

 found that its density was 1*0260, and its temperature 13"7° C. In 

 this water the mackerel ova slowly sank, although the ova of 

 Plenronectes microcephcdus, which I had at the same time, rose to the 

 surface in it as soon as they were introduced. It is remarkable 

 that there should be so considerable a difference in the specific 

 gravity of the ova of the two species, the more so as the mackerel 

 ovum has a large oil-globule, and the merry sole none, and yet the 

 former is a good deal the heavier. As these mackerel ova were in 

 perfectly clean water when brought in I did not transfer them to 



