356 CLASS xvi. 



first rudiment appears on the third day as a vesicular eversion from 

 the posterior extremity of the intestinal canal, becomes highly 

 vascular, rapidly increases in circumference, penetrates externally 

 between the intestine-navel and the amnion, and grows over the 

 embryo and the amnion. About the ninth day the vena terminalis 

 disappears from the vascular area; at a later period its blood-vessels 

 also diminish and disappear. Towards the fourteenth or fifteenth 

 day the allantois coalesces with itself and surrounds the entire egg, 

 being situated immediately under the membrane of the shell; it 

 may now be compared with the external membrane of the mamma- 

 lian ovum, the chorion. The development of the chick still pro- 

 ceeds. From the heart two pairs of vascular arches now arise; the 

 posterior pair already sends many branches to the lungs, and is 

 afterwards changed into the pulmonary artery; the two anterior 

 and an unpaired arch on the right side, between the anterior and 

 the posterior arch, are the principal branches of the great artery 

 present in the full-grown bird, namely the two anterior arteries 

 and the descending artery (see above, p. 338). The permanent kid- 

 neys and the sexual organs are developed. The skeleton becomes 

 more and more complete in all its parts, and in the cartilages from 

 which it is formed many ossific points are now visible. The limbs 

 which at an earlier period were uniform, now gradually assume the 

 appearance which afterwards distinguishes them, the anterior that 

 of wings, the posterior of legs. As early also as the eleventh or 

 twelfth day rudiments of feathers may be observed. The yolk-sac 

 contracts, and at last is, for the most part, received into the abdo- 

 minal cavity, together with the part of the intestinal canal which 

 is on the outside of that cavity, and with which it is in con- 

 nexion by the vitelline duct. The bird, whose bill usually lies at 

 the obtuse end of the shell, perforates on the nineteenth or twentieth 

 day the membrane of the shell, and is able to breathe in the air- 

 cavity under the shell, which explains the fact that the chick may 

 be sometimes heard to chirp before it chips the shell. At last the 

 young bird breaks the calcareous shell, for which a calcareous, 

 conical excrescence on the point of the upper bill, which after- 

 wards disappears without leaving a vestige, serves it as an instru- 

 ment. It enlarges the aperture thus made, and continues still for 

 some time with outstretched neck to rest and breathe in the egg. 

 The circulation of blood in the allantois stops ; the navel-ring closes 



