122 



from 43 to 45 F. (6'l C. to 7'2 C.), and at this temperature the ova would probably 

 take between two and three weeks to hatch. 



It is certain that the eggs of the sole do undergo the same development at about 

 the same rate in their natural condition as they do in experimental conditions. For 

 the characteristics of the sole's ovum having been observed, namely, the size, the layer 

 of segments in the yolk, and the groups of minute oil globules, it is possible to identify 

 ova found in the sea possessing these characteristics as those of the sole. The surface 

 waters of the sea off the coasts of Devon and Cornwall contain nearly all the year round 

 vast numbers of buoyant fishes' eggs. These can very easily be collected by slowly 

 towing through the water from a boat a conical net made of muslin or silk bolting 

 cloth. Among the eggs so collected those of the sole are found in small numbers in 

 March and April, sometimes at the end of February or beginning of May, but at no 

 other time of the year. 



The development of the sole within the egg is rather slow compared to that of 

 some other flat-fishes. The eggs of Pleuronectes flesus, the flounder, hatched in my 

 hatching jars in six days at a temperature of 50 F. (10'0 0.) ; the eggs of the plaice 

 (Pleuronectes platessa) hatched in 10 days at the same temperature; those of 

 Pleuronectes microcephalus, the merry-sole, hatched in eight days at a temperature of 

 about 49-l F. (9-5 C.). 



I have never found the ova of the common sole very numerous among the pelagic 

 ova collected by means of the tow net. The largest number I have obtained at one 

 time is six : these were taken a little to the south-west of the Mewstone. 



The density of the water in which soles' ova are found suspended at sea varies 

 slightly in different places. Examples of water from the sea to the south of the Wolf 

 Eock I have found to be 1*027 (distilled water being 1 ? 0), while samples from the sea 

 off Plymouth, inside the Eddystone, have a density of 1-0267 or 1'0268. I found in 

 my experiments that the sole's ova frequently sank in the aquarium water towards the 

 close of their development, that is shortly before they hatched, and that the larvse 

 lay on the bottom of the jar after hatching : yet the density of this water was 1'027. 

 This shows that the eggs become heavier as development advances, though the 

 sinking of the eggs is probably hastened in the hatching apparatus by the accumu- 

 lation of particles of sediment upon them. The eggs themselves certainly float in the 

 sea until they are hatched, for they are frequently taken in the tow net when just 

 ready to hatch. 



I have not been able to keep the larvse alive mo-re than a day or two after hatching. 

 In fact, owing to the difficulties of artificial fertilisation I have never had a sufficient 

 number to experiment with. I have been equally unsuccessful in procuring the larvae 

 from the sea: it is probable that soon after hatching the larvse sink towards the 

 bottom. Some of the stages given in the illustrations to this book of the larvae of 

 other species of flat-fishes show that at first one of the most obvious changes which 

 takes place after hatching is the absorption of the yolk. Nourished by the yolk thus 



