1 6 BASIC METHODS FOR EXPERIMENTS 



Tempo of development as criterion for normality 



The chief value of knowing the tempo of normal develop- 

 ment for a given tg^ lies In the fact that the Investigator can 

 quickly ascertain at every stage departures from the normal. 

 As the rate of membrane-separation mentioned above serves In 

 the case of certain eggs as a diagnostic of their normality, so 

 the rate of cleavage and of subsequent phases gives information 

 concerning normality. Any departure from what Is known to 

 be the normal rate of development of eggs, whose fertilization 

 has been normal, points to some change Induced by the observer. 



Temperature 



It frequently happens that a worker begins with perfectly 

 normal eggs which deteriorate during the course of development 

 because of abnormal temperature conditions. Some fluctu- 

 ations are bound to occur, but these should be reduced as far 

 as possible. Further, unless eggs are normally found in sea- 

 water of low temperature, they should never be kept in the cold 

 except for the expressed purpose of investigating the effect of 

 low temperature on an egg whose normal habitat Is at higher. 

 I have known many investigators who have kept their animals 

 on Ice over night In order to delay shedding. This practice, I 

 think, can not be too severely condemned. Finally, in making 

 observations on the normal development of an egg, from the 

 container in which the eggs are kept the worker should take 

 samples throughout the course of the observation and should 

 not be content to observe only those eggs transferred to a small 

 glass-container at the beginning of the course of observation. 

 In the small watch-glass or on the microscopic slide, the eggs 

 suffer from change In temperature and may also be affected by 

 increase In salinity due to evaporation. This same consider- 

 ation holds for experimenting on eggs that are In too small a 

 volume of sea-water. 



Evaporation 



Not only should changes In temperature be guarded against, 

 but also changes in salt-concentration of the sea-water brought 



