DEVELOPMENT OF ANIMALS. 647 
layer are destined to arise the skin and wall of the body 
with the nervous system; from the second (mesoderm, in 
the embryo called the mesoblast) are formed the heart and 
the vascular system, as well as the stomach and intestines. 
The middle layer now thickens, causing the mark known 
as the ‘‘ primitive streak,’’ along the middle of which runs 
. the ‘‘ primitive groove.’’ The notochord now appears and 
the muscle-plates (called protovertebra, Fig. 540). The am- 
nion arises as a membrane, splitting off from the outer germ- 
layer of the embryo, and finally forms a cavity which is 
filled with a fluid. About this time the allantois arises as 
an offshoot of the alimentary canal, budding out at the 
hinder end of the embryo, and finally curving over the em- 
bryo, serving as a foetal respiratory membrane. 
Second Day.—The three portions or vesicles of the brain 
now appear (Fig. 540), as well as the alimentary tract and 
heart, both arising in the head-fold or enlargement (Fig. 540, 
a to c), and soon the blood-vessels arise as channels in which 
blood-corpuscles appear, originating as ameeba-like cells 
separating from the cellular mass of the mesoderm. Dur- 
ing the second day also the eyes and ears begin their devel- 
opment, being at first simply folds or inpushings of the 
outer germ-layer. 
Third Day.—This is one of the most eventful days, as im- 
portant steps in the elaboration of the different organs are 
taken ; the different parts of the brain, of the alimentary 
tract and its appendages being sketched out, and the rudi- 
ments of the lungs, the liver, pancreas, nose, and different 
parts of the eye and ear appearing. On the fourth day the 
wings and legs grow out, appearing first as flattened buds. 
The notochord, which is indicated by the second day, by the 
sixth begins to diminish in size, disappearing by the time 
the chick is hatched, while by the twelfth day the deposition 
of bone in the bodies of the vertebree commences. Between 
the eightieth and one hundredth hour the internal differences 
in the sexes appear, the testes beginning to arise on the 
sixth day. 
Fifth Day.—The limbs have by this time developed so as 
to show the knee- and elbow-joints, as well as the cartilages 
