Y82 Part LIT. — 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. Hach 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 fiow 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 were 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), ¢.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. (183 inches) and weighing 1324 
grammes (2 lbs. 14? ounces), and a male of 43cm. (17 inches) and weigh- 
ing 842 grammes (1 1b. 132 ounces). In tank No, 2 the female measured 
46°5cm. (187 inches) and weighed 1536 grammes (3 lbs. 6; 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 auiet in a corner of the tank. In No, 
5, for example, the female lay with her snout as far as possible into the 
