324 ANATOMY OF THE DOMESTIC FOWL 



Of the appendages of the eye the membrana nictitans is the first 

 to develop. Then develop the lower lid and, last, the upper lid. 



After the development of the essential organs of sense, the skin 

 is developed. Modifications of the skin form the outer ear and the 

 eyelids. Then are formed the maxUlary arch, the hyoidean 

 arch, and the scapular arch. 



The vessels which return the blood from the yitellicle are the 

 transverse and the 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 canals. The longitudinal veins 

 extend parallel with the axis of the embryo; they are of smaller size. 

 The right anterior longitudinal vein becomes the right precaval and 

 receives the remains of the right transverse vitelline vein, as the 

 right vena azygos. The left anterior longitudinal vitelline vein is 

 also persistent as the left precaval, and enters in the mature bird, 

 as in the embryo, at the posterior or the lower part of the auricle. 

 The left transverse vitelline vein is also subsequently reduced, by 

 receiving only the vertebral veins of that side, to the condition of a 

 so-called azygos vein. The main trunk of the post-caval is the result 

 of the returning vessels from the abdominal viscera and the posterior 

 limbs at a later stage of development. There is but one principal 

 posterior longitudinal vitelline vein, and this anastomoses with the 

 left transverse vein as it enters the embryo. 



The auricle, which, by its dilatation of the left side, appears to be 

 double, receives the venous blood at its right division. The left 

 one, subsequently receives the veins from the lungs, is ultimately 

 separated from the left precaval and the right auricle to which that 

 vein is conducted and restricted. 



The ventricular part of the heart, at the second day of incubation, 

 is in the form of a bent tube, curving from behind downward, for- 

 ward, to the right and upward, continuing insensibly into the part 

 representing the aortic bulb, in which the septum first appears, and 

 ultimately dividing the ventricle into two. 



At this stage the piers of the maxillary arch appear as buds 

 from beneath the eyeballs. The naso-premaxillary process is above 

 their interspace. The piers of the mandibular arch and those of 

 the hyoidean arch follow in close succession. The blastemal base 

 of the scapular arch projects sUghtly 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 mesen- 



