26 REPRODUCTION AND DEVELOPMENT OF TELEOSTEAN EISHES 



ing, that is to say, as usual in species of fisli tliat swim in slioals and 

 have migratory pelagic habits, is approximately simultaneous in all 

 the specimens in a given locality, proceeds very rapidly when once 

 begun, and is limited definitely to one short period of the year. 



Physical conditions during develoiwient. — I have collected a few 

 data concerning the density and temperature of the water at the 

 surface of the sea, some miles from the Sound, that is where 

 mackerel ova are shed under natural conditions. 



March 23, 1888. — Water brought iu from 2 miles outside the breakwater 



Density . . 1-0268 



Temp, when tested 8'2° C^ 



April 7. — Water from 7 miles south of Wolf Rock . Density . . 1-027 



Temp, when tested 7*5° C. 

 June 1. — Water in which mackerel ova were floating when brought in; that is, 

 water taken from the surface at the place where the mackerel were caught. 



Density . . 10267 

 June 2. — Water in which mackerel ova were brought in from sea. 



Density . . 1-0268 



July 17.— Ditto ...... Density . . 10269 



Temp, when tested 15"0° C. 

 May 31st. — Temperature of the sea 2 miles outside the breakwater. 



Surface . . 9-44° C. 



Bottom, 12 f nis. . lO-Q" C. 

 Jane 12. — Temperature of sea at 7 miles south-west of the Eddystone, taken with 



Casella's reversing thermometer in the "Scottish" frame. Surface . . 11-6° C. 



Bottom, 40 fms. . 10-0° C. 

 July 10th. — Temperature of sea in middle of Plymouth Sound, Melampus Buoy. 



Surface . . 13-3° C. 



Bottom, 7| fms. . 12-7° C. 



With regard to the temperature of the sea at places where 

 spawning mackerel were caught, that is in the neighbourhood of 

 the Eddystone, it is certain that it could not have been higher than 

 the temperature in the Sound and a short distance south of the 

 breakwater. This temperature therefore between June 12th and 

 July 10th rose from the temperature observed at the mackerel 

 ground on the former date, namely, 11*6° C, to something less than 

 that observed in the Souud on July 10th, namely, 13*3° C. With 

 regard to density, the specific gravity of water from the mackerel 

 ground was ascertained on shore three times, and was 1*0267 on 

 June 1st, 1-0268 on June 2nd, and 1*0269 on July 17th. The 

 temperature of the sample at the time when the density was observed 

 was only once ascertained, on July 17th, when it was 15*0 C. The 

 temperature of a small quantity of water in a bottle carried on a 

 fishing boat, and examined in the Laboratory in June and July, 

 must of course be somewhat higher than its temperature when 

 taken from the sea, but we see that the greatest difference of 



