MANUAL OF FISH-CULTURE. 217 



of the milt they are cleaned aud the superfluous milt washed off by 

 introducing a gentle stream of water into the bag and rolling the eggs 

 from side to side. 



It frequently happens that fish held in tanks to mature deposit their 

 eggs during the night. In such cases the eggs are found on the bottom 

 of the tank the next morning. They are usually in clusters and when 

 examined with the microscope it will be found that practically every 

 egg is fertilized. 



After the eggs have been taken aud fertilized the number is calcu- 

 lated by measuring in a glass graduate and computing 47,8_'G eggs 

 to the liquid ounce. The average number of eggs is about 500,000 to 

 a fish. On March 6, 1897,30 ounces, or J, 462,000 eggs, were taken 

 from a fish 20 inches long and 11 inches wide, its weight being 3^ 

 pounds after the eggs were taken. 



Flatfish eggs may be hatched in several kinds of apparatus, but the 

 Chester jar is most used, in combination with the McDonald tidal box 

 employed in incubating cod eggs. From 400,000 to 500,000 eggs are 

 usually placed in each jar. The top of the jar is covered with cheese- 

 cloth held in place by rubber bands. The jar is then inverted and 

 placed in a tidal box. The usual complement of each box is 2 jars. A 

 wooden frame of 1-inch strips is placed lengthwise on the bottom of 

 the box for the jars to rest on, so as to raise them and allow the free 

 circulation of the water. A hole in the bottom of the jar allows the 

 air to pass in and out as the water inside rises and falls. The inner 

 compartment, with a bottom of cheese-cloth, used in cod-hatching is 

 omitted. 



As in using the jars the eggs are generally on the bottom all the 

 time, the experiment has been tried of employing the McDonald box 

 with the automatic current in order to keep the eggs in circulation. It 

 having been found that the current commonly used for cod eggs caused 

 the eggs to pile up in the end nearest the outlet, a stream was intro- 

 duced into each end of the box and the water was allowed to escape in 

 all directions through a perforated nozzle; the water was kept about 3 

 inches deep in the bottom of the box by using a quantity sufficient to 

 prevent the breaking of the siphon. By this means a constant current 

 is formed, the eggs develop nicely, and the fry hatch, but the current 

 necessary to keep the eggs in circulation is strong enough to kill the 

 fry by forcing them against the sides of the box. This experiment is 

 therefore not considered a success. 



The period of incubation when the mean water temperature is 37° or 

 38° F. is 17 or 18 days. 



The fry of the flatfish, although much smaller than those of the cod, 

 are much more lively, and are straightened out when first hatched. 

 Unlike the young cod, they do not float on the surface, but are scat- 

 tered through the water from to]) to bottom, many being seen among 

 the eggs on the bottom of the jars. Unlike the adults, the flatfish fry 



