30 REPRODUCTION AND DEVELOPMENT OP TELEOSTEAN FISHES 



tanks in the mam laboratory was 14"5° C. The temperature of the 

 open sea at this time was not much above 12° C. Thus, in this last 

 experiment with mackerel ova the temperature was only about 2 

 higher than that of the open sea, and the density was 1"0250 or 

 1-0260 instead of 1-0267. 



Some of these ova hatched on July 4th, on the sixth day after 

 fertilization, but these were few in number, all the rest being dead ; 

 the larvae were also half dead and had not strength enough to survive 

 more than a few seconds when placed on a slide ; consequently I 

 was unable to get a drawing of them. On July 5th there were 

 neither larvee nor ova left alive. I got another sample of mackerel 

 ova subsequently, but they were only partially fertilized and 

 soon died. 



It is difficult to say whether the death of the ova in tlie experiment 

 conducted from June 29th to July 5th under the most favorable 

 conditions, was partially due to the presence of impurities derived 

 from the new apparatus, but it has generally been observed that 

 in a new aquarium the animals die in considerable numbers notwith- 

 standing all care. It was so to a certain extent in ours, — there were 

 more deaths at first than subsequently ; and the fact of the system 

 having been so newly arranged may have had an unfavorable effect 

 on the mackerel ova ; but I believe the chief cause of failure was 

 the insufficient density of the water. Apart from the question 

 whether buoyant ova will develop normally in water of such low 

 density that they sink in it, it is certain that in the apparatus used 

 for floating ova the conditions become unfavorable if the ova sink. 

 They are insufficiently supplied with oxygen, and it is impossible to 

 separate the dead ova from the living. Some pelagic ova have been 

 found near or on the bottom in the Baltic, where the density of the 

 water is below that of the open seas. The observations I refer to 

 were made at Kiel by V. Hensen,* a member of the Commission for 

 the Investigation of German Seas, and they refer only to plaice, 

 flounder [Pleuronectes Jiesus), dab [PL limanda), and cod. By 

 fishing with a fine net attached to a dredge at the bottom, at a 

 depth of nine fathoms, about eighteen miles from Kiel, at the mouth 

 of Kiel Bay, he obtained ova of the three species of Pleuronectes 

 mentioned, and these afterwards hatched in captivity. The specific 

 gravity of the water at the place mentioned has an average for the 

 year of 1-0128, and the maximum observed during several years was 

 1*0201, and the average temperature in April, when the ova were 

 taken, was 6-11° 0. Hensen found that the ova of the plaice, after 

 baving been shed into sea- water had a specific gravity of 1-01496 ; 



* Ueher das VorJcomnien und die Menge der Eier einiger Ostseejische, 4ter Bericht 

 der Comiuiss. zur Unters. dei- deutschen Meere, lite Abtheil., 1883. 



