THE PATH OF EVOLUTION 



source, provided that the other necessary conditions 

 of fresh air without excess of moisture are also 

 present. The loss of weight during incubation is 

 about twelve per cent., of which the greater part is 

 due to the evaporation of water. The absorption of 

 oxygen nearly compensates for the loss of carbon 

 and hydrogen by oxidation, and exosmose of one 

 portion of the plasmodium in raising the remainder 

 to the higher organization of the living chick. 



The progressive changes in the egg under the heat 

 of incubation have been frequently studied. At the 

 end of three hours the cicatricula has increased from 

 .006 to .008 m, the transparent centre to .003 m.* 

 In six hours the cicatricula had become .0085, the 

 centre .0035. The embryo has a length of .001 m. 

 In nine hours the cicatricula measures .009 ; the 

 pellucid area .004 m. The shape more decidedly oval 

 and a structural texture more evident. The embryo 

 now is .0027 m in length and its marginal surround- 

 ings better defined. In sixteen hours the disk con- 

 taining the embryo shows great change. The upper 

 lateral surface is much contracted by becoming 

 rounded, and the folds that the membrane has made 

 in thus changing are bent back like a vail before the 

 cephalic extremity of the embryo. Below, the sides 



* A Meter is about forty inches. A millemeter (.001) is approxi- 

 mately one twenty -fifth of an inch. 

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