THE LIFE-HISTORIES OF FISHES 165 



any rate each egg soon becomes surrounded by 

 a number of spermatozoa which adhere to their 

 capsules. By and by one of these reaches the 

 micropyle, and immediately passes through this 

 aperture in the capsule into the protoplasmic 

 germinal disc. As soon as this has happened the 

 micropyle closes, so as to prevent the entrance of 

 other spermatozoa. The egg is now impregnated 

 or " fertilised," and development of an embryo 

 begins. The germinal disc divides to form a 

 number of cells or " blastomeres," and grows until 

 it forms a skin enveloping the whole yolk-mass. 

 On this skin the body of the embryo is formed, 

 and in the course of a few days the young fish 

 becomes apparent. First of all, the spinal cord and 

 brain are formed, and round this central nervous 

 system the whole structure of the little animal 

 becomes built up. After a variable incubation 

 period, the embryo hatches out from the egg- 

 capsule. This incubation period depends first of 

 all upon the species, and is usually from six days to 

 about three weeks. But it also depends upon the 

 temperature of the sea-water in which development 

 takes place, and by altering the latter the egg of a 

 flounder, for instance, may hatch out in from five 

 to ten days. I am speaking just now of the small 

 eggs of the bony fishes. The large egg of a skate 

 or dog-fish pursues much the same course, but the 

 incubation period in these fishes is prolonged, and 

 may extend over the greater part of a year. In 



