SECT. 4] HEAT-PRODUCTION OF THE EMBRYO 



621 



sketched out by Pott & Preyer. Thus they found that the fertiHsed 

 egg developing with the embryo inside it lost on an average 19-6 per 

 cent, of its weight, and that, if the egg was infertile and was yet 

 incubated, it lost nearly as much (18-5), the difference being about 

 I per cent. This coincides with the fact now definitely known that 

 the main source of weight loss in 

 incubating eggs is the evapora- 

 tion of water, not more than 2 or 

 3 gm. of solid being burned away. 

 The earliest observer to note that 

 the weight loss of fertile and in- 

 fertile eggs was much the same 

 appears to have been Erman, who 

 announced it in 1810 in a letter 

 to Oken. Pott & Preyer also made 

 the correct observation that the 

 loss of weight during the incuba- 

 tion period was constant for each 

 day. This, of course, made it ob- 

 vious that the weight loss was 



Fig. 105. 



not directly connected with the 

 embryonic growth, the course of which was known by them to be 

 curvilinear. Fig. 105 is a modification of the illustration they gave 

 of their findings. 



The weight of water W, they said, evaporated each day by the 

 egg as far as the end of the second week, is equal to the total loss 

 of weight, G, for during this time the weight of carbon dioxide 

 produced, K, is exactly equivalent to the weight of oxygen absorbed, 

 S — other gases being omitted on account of their small quantity. Thus 



G = r+ W-S and G^W if K ^ S. 



Roughly speaking, these relationships are still true. 



The question of whether any other gases were given off or taken 

 in during the incubation period by the egg was also handled by 

 Pott & Preyer. Schwann had found that not only carbon dioxide, 

 but also hydrogen and nitrogen, were given off, a result which neither 

 Baumgartner nor Pott & Preyer could confirm. The work which 

 was done later on this point, and which proved that carbon dioxide 

 is the only gas evolved by the developing egg, will be referred to 

 presently. 



