American Fisheries Society 97 



with a common apothecary's grachiate showed no n()ticea1)le 

 the size of the eggs. 



RAINBOW AND STEELIIEAD TROUT. 



With a \'iew to ascertaining the increase in the size of rain- 

 how trout eggs, careful measurements were made several 

 times during the period of incuhation of a particular lot of 

 eggs at the Manchester, Iowa, station during the winter of 

 1906. These measurements demonstrated a slight increase 

 from the time of water hardening until the eighteenth day — 

 324 temperature units — and then a slight decrease until the 

 thirty-second day — 516 temperature units. It is the opinion 

 of Superintendent Johnson, who made these interesting 

 observations, that there is a slight increase in the size of rain- 

 bow eggs fmm the beginning of segmentation until the eye 

 spot clearly appears, which usuall}' takes place at about 285 

 temperature units. After this time there is a gradual de- 



Until a development of about 396 temperature units is 

 reached the mem1)rane of the egg is firm, after which it so 

 increases in softness and tenderness that it cannot l)e 

 handled without danger of breaking. At this period tlie egg 

 is very susceptible to slight pressure, its own weight when it 

 is taken from the water tending to a pronounced flattening. 

 It is largelv as a result of this softening that it loses the uni- 

 form!}' s])herical contour of the earlier stages, and becomes 

 more or less elongated and irregular. In the advanced or 

 soft stage the eggs naturally lie very close together when 

 measured in a graduate and this possibly accounts for the 

 apparent slight decrease in the size just before hatching. 



In addition to determining results by the ordinary grad- 

 uate measurement, Mr. Johnson drew his conclusions from 

 a measurement of the bulk of the eggs by water displacement 

 in a graduated glass tube. This method he considers to be 

 much more accurate, especially in its application to this 

 species of eggs. 



