THE BIOLOGY OF THE CELL SURFACE 



which eggs in optimum physiological condition vary must 

 also be known in order that differences discovered can be 

 truly attributed to the experimental means. 



These few examples show how careful we must be in 

 drawing conclusions from experiments in biology. The 

 highly irritable systems that we investigate demand a most 

 exact control of the experimental factors. If only one of 

 these is left aside in our evaluation of the obtained results, 

 our conclusion may be wholly erroneous, especially if the 

 factor induced a change at the beginning of the treatment. 

 We must know whether we have before us normal or already 

 changed, abnormal, eggs. We must control the effect of 

 our means, even of those that are only aids, by following 

 their effect singly before we use them together in one experi- 

 mental setting. And above all must we be wholly familiar 

 with the normal condition and normal development of the 

 eggs we use, since only on this basis are we able to recognize 

 abnormalities and thus the effect or effects of our experi- 

 mental treatments. 



The simplest and most generally adopted criterion for 

 the eggs' normality lies in estimating the number of them 

 that develop; one hundred per cent, development means 

 optimum condition. Eggs that are laid already fertilized, 

 and especially those in brood pouches or in capsules almost 

 always develop normally. Also where the eggs and 

 spermatozoa are deposited and fertilization ensues in the 

 laboratory as in nature, the chances for normal develop- 

 ment are enhanced. It is with those eggs whose deposition 

 is induced by the experimenter and which are artificially 

 inseminated and especially with those that are removed 

 from the ovaries that most difficulties are encountered. 

 In many cases such eggs develop, if at all, only in small 

 numbers. They are only useful for studies on development 

 if the cause for the low numbers of development can be 

 discovered and the percentage increased. There are cases, 



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