CHAPTER IV. 
STRUCTURE AND DEVELOPMENT OF THE EGG. 
B eyond the fact that an egg consists of yolk, white, and shell, little is popu- 
larly known respecting its structure ; and even a less amount of knowledge 
prevails regarding the changes that occur in it during the development of the 
embryo. In these, as in all other cases, ignorance leads to serious errors in 
management, and consequently to severe losses in practice. We have therefore 
deemed it desirable to enter somewhat more fully than is usually done in popular 
works into the consideration of this interesting subject. 
The first part that claims our notice is the shell. This, which usually weighs 
about one-tenth of the entire egg, is formed of carbonate of lime, the same sub- 
stance that, in a different state of aggregation, constitutes marble, chalk, and old 
mortar. The materials of the egg-shell do not, however, form a uniform layer, 
but are arranged in such a manner as to leaves pores, or minute apertures, through 
which the moisture of the egg can evaporate, and the external air gain entrance 
to support the breathing of the unhatched chick. The particles of carbonate 
of lime are arranged in somewhat the same manner that the bricks are placed in 
a dome or arch ; by this means enormous strength is given to the shell. It may, 
perhaps, surprise many persons to be told that the strongest man cannot crush an 
egg if his strength be fairly and uniformly applied. This experiment is easily tried 
by placing an egg endways between the palms of the hands it will be found that 
the greatest exertion fails to crush it when it is fairly placed and evenly pressed 
from end to end. As this strength, however, would render the exit of the young 
chick a matter of great difficulty, an exquisitely beautiful provision of Nature is 
arranged in order to permit it to find its way out of 
“ that antenatal tomb, 
■Where! the young bird dreams of the life to come.” 
The structure of the shell that gives its great strength to the fabric is entirely 
disarranged during the process of incubation ; at the period of hatching the 
particles of carbonate of lime are confused together, and have lost all their regu- 
larity of arrangement ; hence the egg-shells at this time become very brittle, and the 
exit of the young bird is easily accomplished. So different is the arrangement of 
the particles in the shells of hatched and unhatched eggs that they can readily be 
distinguished by microscopic examination ^ It is this peculiar change of structure 
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