THE BEHAVIOR OF BROOK TROUT EMBRYOS 



47 



that a slit appears. There does not seem to be any distinction 

 as to which part of the embryo comes out first, for in the twenty- 

 three cases observed, eight embryos appeared head first, one 

 tail first, and in fourteen cases the yolk sac broke through before 

 the body. The final shedding of the egg case is sometimes brought 



Fig. 1. Egg of a Brook Trout shortly before hatching. Magnified six and 



two-third times. (Drawn by Miss Wakeman) 



Fig 2. Embryo hatching head first. Magnified six and two-third times 



about by a violent movement of the body, but in nearly all the 

 instances observed it was a gradual process lasting from three- 

 quarters of an hour to five or six hours, during which the initial 

 slit was slowly enlarged by the rhythmical motions of the body 

 and the respiratory movements. If the anterior end is to be 



^Z^ 



Fig. 3. 



Dorsal view of an embryo three weeks old (15 mm.). Magnified 

 six and two-thirds times 



the first to appear, the violent contraction of the embryo raises 

 the head until the strain splits the shell far enough to free the 

 region bearing the pectoral fins, which immediately begin to 

 move. The front of the head soon follows. Whether the tail 

 or the yolk sac comes out first, the length of time required is 



