Birds' Eggs during Incubation. 139 



per ceut. In a letter published later in ' The Ibis/ 190J;, 

 p. 662j Mr. Nevin Foster stated that the decrease in the weight 

 of Thrushes' and Blackbirds' eggs during incubation is about 

 15 per cent. My results shew that, in like manner, hens' 

 eggs lose weight during incubation to the extent of between 

 14 and 15 per ceut. For example, the average Aveight of 

 12 eggs descended from 57*66 grammes to 49'23 grammes 

 in 20 days : this is an average decrease of 14*58 per ceut. 



Apparently all the eggs weighed by Mr. Gladstone 

 contained living chicks. Now in each of my batches of 

 incubated eggs some proved infertile and others contained 

 dead embryos of varying ages. The curious and interesting 

 fact is, that in every case the infertile and dead-embryo- 

 bearing eggs decreased in weight in very nearly the same 

 degree as did the eggs which eventually hatched out into 

 sturdy chicks. A line which shews the average decrease in 

 weight of three clear, infertile eggs is nearly parallel to 

 that line which shews the average weight-decrease of three 

 eggs — incubated at the same time — from which emerged 

 three perfectly developed chicks. 



On examining the lines shewing the loss of weight of 

 individual eggs, I noticed another interesting point — a line 

 generally makes a sudden, erratic drop just before that spot 

 which marks the death of the embryo. Thus, an egg which 

 at the close of the period of incubation contained a dead 

 embryo appears to have suffered an increased loss of weight 

 when the embryo was dying. 



The chemical examination and explanation of the loss in 

 weight of incubated eggs, and especially of the fact that the 

 weight-lines of dead and living eggs are so nearly parallel, 

 would prove of great interest. I found that the infertile 

 eggs had not become at all putrid at the end of three weeks' 

 incubation: apparently they lost weight by losing water 

 only. The living eggs, one might assume, would lose water 

 to the same extent as the dead ones, for they were kept 

 under identically similar conditions ; but the eggs containing 

 living chicks have the Aveight of the products of respiration 

 and excretion to be accounted for. 



