48 GERTRUDE M. WHITE 



nearly the same, and the details of the process differ very little. 

 Unless the animal is disturbed, it may lie quiet for hours while 

 the egg case slips off, but if the fish is jarred, a violent contrac- 

 tion almost invariably results, thus hastening the loss of the 

 egg case. In nature, where there are great numbers of fish 

 hatching simultaneously, they undoubtedly touch one another 

 continually, shortening the time required for hatching. 



The Brook Trout makes its appearance as a pretty, delicate, 

 translucent creature about twelve centimeters long. Its most 

 conspicuous features are its enormous eyes, which occupy almost 

 the whole head, its huge yolk sac covered with a fine network 

 of bloodvessels leading to the heart, the beating of which can 

 plainly be seen. Along the back is a strip of pigment extending 

 from the head to the tip of the slender tail. 



Fig. 4. Lateral view of an embryo three weeks old. Magnified six and 



two-thirds times 



2. Swimming Movements 



Although the Brook Trout spends most of the first six weeks 

 of its larval life lying quietly on its side, it is perfectly capable 

 of swimming as soon as the egg case is lost. In fact, Paton 

 ('07), who worked on trout among other varieties of fish, states 

 that definite progression is possible when the embryo is nine 

 or ten millimeters long. In the present experiments, if a fish 

 which had just hatched was suddenly touched, it would whirl 

 round and round as if its cumbersome yolk sac formed a movable 

 pivot. By the fourth day the movements are still rotatory, but 

 the fish swim in larger circles and can go straight ahead for a 

 greater distance. There is also more darting about, the tail 

 being always the most active part of the body. The trout which 

 is one week old (14-15 millimeters) swims in a spiral course. As 

 the yolk sac diminishes in size, the fish is better able to control 

 its movements. It lies upon its ventral side like an adult at the 

 age of six weeks in most cases, and in swimming, continually 

 darts about, turning first one way, then the other. 



