end of incubation, may be considered as coming under this 

 caption. Such conditions as are quoted in nearly every 

 poultry raiser's manual can be mentioned here ; an attentive, 

 faithful setting hen may bring out a hatching of chicks in 

 twenty days (33), and favorable weather and a suitable nest 

 site (104) also tend to maintain optimum temperature con- 

 ditions, with possibly a true, but very slight, shortening of 

 the incubation period. 



This true variability towards the side of shortening the 

 period of incubation is probably in progress now, the world 

 over, with many different species, especially the higher 

 birds, and it will be taken up in greater detail later on. 



Apparent variability By apparent variability I would 

 have understood all lengthening of the specific incubation 

 period which is merely an addition to it of days of pro- 

 longed and retarded embryonic development caused by the 

 various factors mentioned in this discussion, or seeming 

 shortening or lengthening due to error of determination. 



Apparent or false variability is, in the greatest number 

 of cases, merely the result of cooling the egg during incu- 

 bation, which slows down the developmental pace, or it may 

 actually suspend it for a while; in fact, the developmental 

 process "can be suspended and held in check for several days 

 without destruction of the germ" (33). There can be no 

 question as to the effect of cooling the eggs while they are 

 being incubated, because it has been proven many times 

 (by accident or design) with the domestic hen (33) that its 

 period of incubation can, by such cooling, be extended to 

 twenty-three or even twenty-four days. It is probable that 

 eggs of the lower birds can be chilled a much longer period 

 than can those of the higher species, without killing the 

 embryo, a fact which probably helps to explain the seem- 

 ingly great variability shown in the incubation records of, 

 for example, the emu. 



There are many ways by which this qooling action 

 occurs : a restless, inattentive hen, a cold site for a nest, con- 

 ditions preventing the eggs from receiving the necessary 

 heat properly, as too thick shells, or which permit too rapid 

 radiation, as too thin shells, or a poorly constructed nest; 

 eggs which are too small radiate their lieat too quickly, on 

 exposure, since they are relatively of larger surface area 

 than are larger eggs; parents in ill health or badly nour- 

 ished will not produce optimum temperature conditions. 



It appears to me that all of the conditions which are 

 outlined in this study, which apparently modify the true 

 or specific length of incubation, should be taken into account 

 in the future in all field, incubator and zoologic park work, 

 in order that their effects may be eliminated in an endeavor 

 to determine with the greatest possible accuracy the true 



17 



