THE DEVELOPMENT OF BIRDS. 225 
the latter the first rudiment of the back-bone is formed. Then, 
after certain other elevations and depressions and various fold- 
ings, blood is formed, vessels arise, a heart shows itself and beats, 
and a primitive circulation is established. Limbs also grow forth, 
and jaws, and sense-organs form themselves, and so, little by 
little, what was at first a minute spheroidal particle of proto- 
plasm, more and more approximates to the form of a Bird. 
But the body is only built upin a very roundabout way. Its 
earlier structural arrangements are very different from those of 
adult life. The brain is at first more like that of a fish than 
of a Bird. The heart begins as a simple tube, which subse- 
quently becomes bent on itself and subdivided into chambers. 
The blood-vessels which go to and from it are at first very 
different from what they ultimately become. 
The arteries which proceed from it form at first a series of 
arches passing up on either side of the neck to unite dorsally 
and there give rise to the commencement of the aorta. 
. Certain clefts, termed visceral clefts, also exist for a time on 
either side of the throat, while the series of solid parts left 
between them are named visceral arches, along the inside of which 
proceed the arteries just mentioned as arching up on either side 
of the throat. These conditions are very fish-like. 
The skeleton is at first represented only by stretches of 
membrane, afterwards by these and by cartilages, und only 
finally by bones. Instead of the series of vertebre which later 
make up the back-bone, there is at first only a continuous 
gelatinous rod, called the notochord or chorda dorsalis. The 
bones are at first much more numerous than those which are 
found distinct later in life, especially in the cranium. Finally, 
before hatching, a covering of feathers may be formed which is 
very different from that of the adult, and is sooner or later cast 
off. 
At a very early stage of this process a membrane grows up 
around the embryo, and the upgrowths meeting together above 
it, unite and enclose it. This membrane is called the amnion, and 
it is filled with fluid—the amniotic fluid—wherein the embryo 
lies as in a water-bed. 
Another membrane grows forth from the ventral surface of 
the embryo’s body, and spreads out on all sides of it immediately 
within the egg-shell. This is called the allantois, and is the 
bird’s breathing-organ while developing within the egg. The 
egg-shell is porous, and allows air to pass through, while blood- 
Q 
