OCCURRING IN THE NEIGHBOURHOOD OF PLYMOUTH. ]5 



aeration or circulation, but were transferred to clean water every 

 day. On April 14th the temperature of the water in the jars was 

 12'5° C, more than 5° higher than the surface temperature of the 

 open sea where they were taken. The subsequent temperatures in 

 the jars were: 16th, 11-0° C. ; 17th, 13-0° C. ; 19th, 11-8° C. 



A large proportion of this lot of ova hatched, and some of the 

 larvae lived five days after hatching. The larvte after hatching 

 were placed in a jar with gravel at the bottom, supplied with a con- 

 stant inflow of water, the outflow taking place from a cylinder whose 

 base was inserted in the gravel. 



On the 18th I found that water brought up from the shore had 

 a density of 1023, and in this ova of PI. microcephalus rapidly sank. 



The ova fertilized on May 24th were also left in still water, 

 having a density of 1*026, and a temperature on the second day of 

 13*7° C. These hatched on May 30th. Another lot, fertilized on 

 May 29th, were placed in a Chester apparatus (see under Scomber), 

 made with a square wooden washing tray, and provided with a 

 nearly constant supply of water from the shore. (All this time I was 

 restricted to a single room, all water being carried up by hand.) The 

 temperature of the water was 12*4° C. These ova were all dead on 

 May 31st, the fatality being probably due to the impurity of the 

 apparatus ; the washing tray was fastened with white lead, and 

 washed muslin was used over the bottom of the jars containing 

 the ova. 



It is thus shown that the ova of PI. microcephahts are extremely 

 hardy, and can be hatched without any difficulty in still water 

 whose temperature and density differs considerably from those to 

 which the ova are exposed under normal conditions. The next step 

 is to make arrangements for collecting and hatching these ova on a 

 large scale, and transferring the hatched young to the sea, in order 

 to find if the supply of merry soles is thereby increased. 



The specific gravity of the ovum of PI. microcephalus is about 

 1*024 ; they floated in water having a density of 1*025, and sank in 

 that of density of 1*023, 



Structure and developmeiit.-^'-The ovum of Pleuronectes micro- 

 cephalus resembles that of other species of the same genus, and of 

 many species of Gadus (cod, haddock, whiting, &c.) in having a 

 perfectly homogeneous yolk without oil-globules, and a small 

 perivitelline space. Its diameter is usually 1"86 to 1*44 mm., 

 though individual ova may be a little smaller or a little larger than 

 this. The external surface of the vitelline membrane {egg envelope) 

 is not perfectly smooth, but shows a number of fine raised ridges 

 forming two systems of parallel lines, which cross one another 

 diagonally. Fig. 6 shows the appearance of the ovum under the 



