THB DBVELOPMENT OP BIEDS. 225 



the latter the first rudiment of the back-bone is formed. Then,' 

 after certain other elevations and depressions and various fold- 

 ingSj 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-prgans 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 up in 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, and only 

 finally by bones. Instead of the series of vertebrae 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 bf the adult, and is sooner or later east 

 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 



