A METHOD OF RECORDING EGG DEVELOPMENT, FOR USE OF 



FISH-CULTURISTS. 



By Claudius Wallich. 



In watching the development of fish embryos it is observed that the 

 rate of growth of the embryo is dependent almost directly upon the 

 temperature of the water. It has been customary to refer to the vari- 

 ous stages of growth as making their appearance in a certain number 

 of days or hours at a given mean water-temperature. As this mean 

 is obviously subject to great variations, the lengths of time at which 

 given phenomena appear are subject to correspondingly great varia- 

 tions. Then, too, in the embryonic life of each separate species of 

 fish there are a number of definite periods that must be well known 

 to the fish-culturist, for efficient work; such, for instance, as the begin- 

 ning of the critical stage, the ending of the same, the earliest shipping 

 age for eyed eggs, the latest shipping age for a given distance and 

 conditions, the length of the incubation period, and the time required 

 for sac absorption — all very important questions, concerning which 

 accurate and immediate knowledge is often required. Again, the 

 given mean water-temperature for the varying lengths of time at which 

 these definite phenomena appear is not and can not well be computed 

 until the phenomena have actually occurred, and it is therefore impos- 

 sible to closely predict their time of occurrence. 



The complexity of these data, as well as their importance, makes 

 very desirable a simpler, more convenient, and more efficient terminol- 

 ogy expressive of the rate of growth than the one above mentioned. 



During the season of 1898 and 1899, at the United States Fish Com- 

 mission station at Baird, Cal. , there was tested a system of recording 

 egg development which may be termed the "temperature or thermal 

 unit system." By temperature unit (t. u.) is meant 1° F. above 32° for 

 a period of 24 hours. Thus a mean temperature of 36° F. for one day 

 is equivalent to 4 temperature units, etc. One degree centigrade for 

 the same period would make a more convenient unit, as it would do 

 away with the subtraction of 32 each day to find the resultant units; 

 but as Fahrenheit thermometers were in general use the Fahrenheit 

 scale was employed in this case. 



To use this system of recording egg development, subtract 32 from 

 the mean water-temperature of the day the first eggs are taken. This 

 gives the age of the eggs in temperature units on the second day. 

 The temperature units of the second day are added to those of the 



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