Eggs from maxane, Coregonus lavaretus, about 360 day-degrees j at an 

 average temperature of /+ degrees centigrade. 



Here, the tine is figured fron fertilization to beginning of hatching. The hatching 

 was mostly finished after 50 day-degrees. But the number of day-degrees changes with 

 changing degrees of temperature, as Ainsworth (according to von dera Borne) already 

 mentioned with regard to brook trout. Quite recently, Leiner (according to Lange) found 

 that in the case of the stickle-back, the number of day-degrees will drop evenly from 

 230, at a ten5)erature of 8 degrees, to 110 at a temperature of 28 degrees. 



Since my own observations are in accordance with those of Ainsworth, I give here a 

 chart, concerning the incubation period of eg^s from brook trout at different temperatures. 

 The dotted line in this chart pertains to rainbow trout and is sufficient for all practical 

 purposes. Since slight changes in temperatures occur constantly, and other factors also 

 vary slightly, it is impossible to predict with utmost exactness the moment of hatching. 

 The eye spots in eggs of rainbow trout become visible shortly after the first half of the 

 development stage is reached. 



J « 5 5 



9 10 11 12 13 n 15 



"Cntlgrad, 



Fig, 32, Developing period of brook and rainbow trout eggs in 

 different temperatures. 

 Bp: Development period of brook trout up to development of 

 eye points and red blood, 

 B: Development period of brook trout eggs up to moment of 



hatching, 

 R: Development of rainbov; trout eggs up to moment of hatching. 



One day after stripping, the percentage of fertilized eggs can be tested by placing 

 the eggs in Hoffer's solution (I/A. percent of chromic acid.. 3 partsj 10 percent nitric 

 acid. .A parts; 96 percent alcohol. ,30 parts). In the fertilized eggs a separation into 

 2 or /i parts of the white germ field become clearly visible within a few minutes. If 

 the test is made 8 days after fertilization, a streaked outline of the embryo becomes 

 visible. 



The following work is to be done during the incubation period: 



(1) At short intervals (about every two days) the died-off eggs (Fig. 33) are 

 to be removed, in order to prevent contamination of healthy eggs through 

 their decay. This is best done by means of the Lietmann forceps which 



116 



