DEVELOPEMENT OF BIRDS. 2G3 



ries, fig. 136, from tlie aortic bulb traverse the tissue between the 

 hyoidean and scapular arches. 



The channels which return the blood from the vitellicle are 

 the ^ transverse ' and ^ longitudinal vitelline ' veins : the first are 

 so called because these trunks pass to the embryo at right angles 

 to its axis ; they are the largest returning channels : the longitu- 

 dinal veins run parallel with the axis of the embryo and are of 

 smaller size. The right anterior longitudinal vein, fig. 136, j^, 

 becomes the right precaval and receives the remains of the 

 right transverse vitelline vein, ib. s, as the right vena azygos. 

 The left anterior longitudinal ^-itelline vein, ib. p, is also per- 

 sistent as the left precaval, and enters in the mature bird, as in the 

 •embryo, the posterior or lower (sacral) part of the auricle. The 

 left transverse vitelline vein, ib. r, is also subsequently reduced, 

 by receiving only the vertebral veins of that side, t, to the con- 

 dition of a so-called ^ azygos vein.' The main trunk of the post- 

 caval is the result of the returning channels from the abdominal 

 viscera and the hind-limbs, at a later stage of developement. 

 There is but one principal posterior longitudinal vitelline vein, 

 ib. q, Avhich anastomoses with the left transverse vein as it enters 

 the embryo : the homotype of the right side appears as one of 

 the ordinary small tributaries of the right transverse vein. 



The auricle which by the dilatation of the left side, ib. u, 

 appears to be double, receives the venous blood at its right 

 di\dsion. The left one, subsequently receiving the veins from the 

 lungs, is ultimately separated from the left precaval and right 

 auricle to which that vein is conducted and restricted. 



The ventricular part of the heart, ib. v, at the second day of 

 incubation, is in the form of a bent tube, curving from behind 

 downw^ard, forward, to the right and upward, continued insensibly 

 into the part representing the ^ aortic bulb,' ib. f, in which the 

 septum first appears, ultimately dividing the ventricle into two. 



At this stage the piers of the maxillary arch, ib. a, appear as 

 buds from beneath the eyeballs ; the naso-premaxillary process, 

 ib. /, is above their interspace ; the piers of the mandibular arch, 

 ib. h, b, and those of the hyoidean arch, ib. c, c, folloAv in close suc- 

 cession. The blastemal base of the scapular arch, ib. d, d, slightly 

 projects at the sides of the ' fovea cardiaca : ' the piers, now separate, 

 ultimately meet in front of the heart, and accompany it in its 

 retrograde course. The mesencephalon, ib. in, is the largest 

 segment of the brain, in connection with the eyeballs, «, o. 



When the heart has assumed its form, as such, distinct from 

 the great trunks rising from it, the arteries from the base of the 



