FORMATION OF THE CHICK 307 
cavity at the upper end of the oviduct, and there, as the naked 
yolk passes into the oviduct, before albumin has been added, the 
necessary union takes place. 
It is a common but mistaken belief among the inexperienced 
that a male bird in the pen is necessary for the production of eggs. 
This is true so far as the laying of fertile eggs is concerned; and, 
while it is true that the primary object of egg production is repro- 
duction, yet impregnation is unnecessary for the production of the 
egg, and hens will lay just as many eggs when no male bird runs 
with the flock as they will with one. In fact, it is becoming a 
rule on the larger egg farms to produce infertile eggs for market, 
since they are less likely to spoil, and there is no danger of the 
germ developing and ruining the eggs for eating. Fertilization is 
not an incentive to egg production among domestic fowls. 
The egg, then, is a productive sac surrounding a female germ 
cell, which may or may not be fertilized. In the latter case the 
egg is termed infertile and cannot possibly be hatched, because to 
produce life there must be the union of the male and female 
germ cells. 
Formation of the Chick.—The first stage in the development 
of the embryo takes place before the egg is laid, probably due to 
the heat from the bird’s body. This change is termed “ segmen- 
tation,” and results in the multiplication of cells which form a 
cap over the germ vesicle and a group of cells under it. In the 
normal egg, development is arrested at this stage, and a certain 
amount of heat is necessary to renew it after the egg is laid. 
After segmentation and application of the right degree of heat, 
incubation begins. The germinal disk divides into two layers, 
between which a third stratum soon forms. The upper layer 
(called the epiblast) produces ultimately the skin, brain, spinal 
cord, eye, and internal ear. From the lower layer (hypoblast) is 
formed the lining of the digestive tract, while from the middle 
layer (mesoblast) are developed all the other organs, such as bones, 
nerves, and muscles. This middle layer thickens rapidly, forming 
two parallel ridges running lengthwise of the germ, with a groove 
between them which is termed the medullary canal; the walls of 
this groove gradually extend and finally meet, forming a tube or 
neural groove, in which the brain and spinal cord develop later. 
The notochord just below the tube can be distinguished from 
the first day of incubation. It marks out the future bony axis of 
the body, or the vertebral column. From the notochord are 
