VARIABILITY in the development of the chick is an accepted fact, but differences noted between embryos from modern incubators and those of Duval (1889), which have served as the basis for most later tables of chick development (Lillie, 1919; Patten, 1929), are apparently too great to be safely included in the normal range of variation. Those from modern incubators were markedly less developed at various incubation ages, and it seemed unlikely that such discrepancies could be traced to differences in types of incubator alone; therefore, it appeared desirable to calibrate the development of the chick in modern incubators, controlling, as completely as possible, various factors that have been mentioned as possible sources of abnormal variation. Several evident causes of variation have been enumerated and the influences of each have been studied more or less intensively. Older incubators did not possess mechanisms for control of temperature and humidity; ventilation was so . . .
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Joe Kendall Neel (1942) studied this question.