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Part III.— Twenty-fourth Annual Report 



plaice already in one of the papers above alluded to, but by a different 

 method, viz., by counting the eggs in a given portion of the ovary after 

 the death of the fish, and then computing the total numbers. 



The experiment consisted in keeping two female plaice, which were 

 nearly ready to spawn, in separate tanks, with one or more mature males 

 with each of them, until spawning was quite concluded ; to collect all the 

 eggs daily and ascertain their numbers. 



The tanks in which the experiments were made are similar in all 

 respects. Each is 6 feet 4 inches in length, 4 feet 3 inches in breadth, 

 and 3 feet 6 inches high, and the water was kept at a level of 28 inches 

 throughout the period. The quantity of water was thus about 62 cubic 

 feet, or 385 gallons (1749 litres). The flow through it amounted to about 

 80 gallons per hour. The glass front of the tank was covered over with 

 an old sail, and light was admitted from the top, but it was not very 

 strong. The water entering the tanks was filtered through close and 

 thick flannel to obviate the chance of eggs getting into them in that way ; 

 and no other fish or organisms w r ere in the tanks — nothing but the plaice. 



The overflow from each tank was carried into two boxes, each about a 

 foot square, the bottoms of which were covered with fine silk-netting, so 

 that all eggs might be retained ; these were partly immersed in order to 

 keep the eggs living, and the water passed from one of the boxes into the 

 other, being thus twice filtered, an arrangement adopted lest the fine 

 apertures in the silk gauze in the first box should get clogged up during 

 the night. 



The method of dealing with the eggs was as follows : — Each collection, 

 daily, or, usually, twice a day, was preserved in sea-water with a little 

 formaline. The eggs were then strained off and spread on blotting paper, 

 and as much of the water and moisture as possible removed. They were 

 then added to a long burette, graduated to tenths of a cubic centimetre, 

 in which a certain measured quantity of water had been placed. The 

 difference between the readings before and after the eggs were introduced 

 gave the volume of the eggs in cubic centimetres. Some were also 

 weighed on a Sartorius balance. A portion of the eggs in each collection 

 were treated in a similar way and, after the volume had been determined, 

 were counted. The number of eggs per cubic centimetre, and the 

 total quantity of eggs in the collection, were thus estimated. 



Two suitable female plaice were selected from those in the spawning 

 pond (used in connection with the hatchery), i.e., which had swollen 

 ovaries and obviously would soon spawn, but which contained no ripe eggs. 

 This was ascertained by pressure in the ordinary way. On the other 

 hand, males were taken from which spermatic fluid was already oozing, in 

 order to provide that the eggs should be fertilised, and to furnish such 

 psychological stimulus to the female as might be necessary and natural. 



On the 19th February the following plaice were put into the tanks : — 

 In No. 5 a female measuring 47cm. (18| inches) and weighing 1324 

 grammes (2 lbs. 14| ounces), and a male of 43cm. (17 inches) and weigh- 

 ing 842 grammes (1 lb. 13| ounces). In tank No. 2 the female measured 

 46'5cm. (18| iuches) and weighed 1536 grammes (3 lbs. 65 ounces), and 

 was thus heavier than the other. Two males were put in with her, 

 measuring respectively 38:6cm. and 40cm. 



In the course of the experiment, when a pause occurred in spawning, 

 the males were sometimes replaced by fresh ones taken from the pond, as 

 mentioned later. 



No eggs were obtained from either tank until 19th March. During the 

 interval the males and females lay quiet in a corner of the tank. In No. 

 5, for example, tke female lay with her snout as far as possible into the 



