139 
Birds’ Eggs during Incubation. 
per cent. In a letter published later in f The Ibis/ 1904, 
p. 662, 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 cent. For example, the average weight of 
12 eggs descended from 57*66 grammes to 49*23 grammes 
in 20 days : this is an average decrease of 14*58 per cent. 
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 weight of the products of respiration 
and excretion to be accounted for. 
