374 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



space below it. Tliis space between the yolk and the egg membrane is 

 called the perivitelline space. The eggs of all salmon and trout swell 

 up about one-third larger in size than when first laid (fig. 5), and the 

 outer shell becomes so tightly stretched that the eggs are rigid and, 

 when dropped, may bounce like rubber balls. The egg membrane is 

 adhesive when first laid, but at the end of about three-quarters of an 

 hour, after the completion of water-hardening, it is smooth and no 



10 15 20 25 



NUMBER OF MINUTE5 IN WATER 



30 



FiQUBK 5.— The rate o( increase in weight (water-hardening) of the eggs of the little redflsh, Oncorkynehu* 

 nerka. The eggs from six females were used in six different experiments. All weights were converted 

 into comparable terms and are expressed as the average initial weight of all eggs from the six females. 

 This was necessary because the eggs in different females vary greatly in size and weight. Each black 

 spot on the graph represents an average determined from about 70 eggs. The average initial weight was 

 0.0645 gram per egg. 



longer sticky. This adhesive quaUty causes grains of sand to adhere to 

 the eggs wliich helps to keep them in the bottom of the nest until they 

 are covered with gravel by the female. 



POST-SPAWNING ACTIVITIES 



Within a few minutes after the eggs are laid and fertilized during the 

 spawning act, the female tiu-ns on her side and begins to cover them in 

 the same manner that she excavated the nest. This is accomplished 

 not by digging into the pit where the eggs occur, but slightly upstream, 

 so that the current sweeps the distui'bed material over the eggs. 



