THEORY OF INCUBATION. 639 



process of incubation went on without interrup- 

 tion, till the twelfth or thirteenth day, when kept 

 at the temperature of 100; and that the allantoid 

 membrane was perfectly formed.* 



Dr. Southwood Smith maintains, that chyle 

 and venous blood are converted into the arterial 

 state, by parting with carbonic acid and water in 

 the lungs ; in exchange for which, they receive 

 oxygen and nitrogen. Muller also contends, that 



* But in his able Lectures on Generation, (published in the Lan- 

 cet, 1840,) Mr. Owen states, that when a solution of acetate of lead 

 was introduced through an opening atone extremity of the shell, 

 which was then carefully luted, covered over with paper dipped 

 in albumen, and smeared with varnish, as in the experiments of 

 Mr. Towne, patches of black pigment, or sulphuret of lead, were 

 discovered on the internal surface of the shell, after exposing the 

 egg for some time to the influence of sulphuretted hydrogen. He 

 therefore contends that this gas must have found its way through 

 the different layers of paper dipped in albumen, because they 

 were changed yellow ; and, that air may percolate the coating 

 of varnish, and afterwards find its way to the internal parts of 

 the egg through the layers of paper. I have also observed that 

 the blood going to the respiratory membrane is of a dark venous 

 hue, and returns of a scarlet colour, proving that, to a certain 

 extent, it undergoes the same change as while passing through 

 the lungs after leaving the shell. But we are greatly in want of ad- 

 ditional experiments for ascertaining with precision, what amount 

 of carbonic acid is thus generated and exhaled through the pores 

 of the shell during the various stages of incubation, in short, 

 what is gained and what is lost. Wagner states, that during the 

 first week, the egg loses five per cent, of its weight, thirteen per 

 cent, the second week, and sixteen per cent, during the third week. 

 (Elements of Physiology, p. 129, trans, by Willis.) But Mr. 

 David Tod found the loss to vary from ten to fifteen per cent, 

 during nineteen days incubation, whether the eggs were fecun- 

 dated or not. 



