EMBRYOLOGY OF THE CHICK 325 



cephalon is the largest of the segments of the brain which are con- 

 nected with the eyeballs. 



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

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

 ventricles appear. The artery to the right bifurcates, one division 

 supplying the head and the wings, the other winding over the right 

 bronchus. That to the left also bifurcates. Its left division arching 

 over the left bronchus and anastomosing with the right arch a httle 

 below and behind the apex of the heart. Its right division arches 

 over the back of the heart, bending to the right and anastomosing 

 with the right aortic arch just above the outer ductus arteriosus. 

 Each of these divisions of the left primary arterial trunks sends off 

 a branch to its corresponding lung. As the lung expands, and 

 especially at the beginning of the act of expansion toward the close 

 of the period of incubation, the blood is diverted into thepulmonary 

 vessels, and the channels below them shrink and disappear. The 

 left primary artery is retained as the trunk of the puknonaries, and, 

 through the changes in the interior of the ventricle, this artery comes 

 to discharge exclusively the ventricle corresponding to the right in 

 mammals. The retained aorta rises from the left ventricle. 



The air-sacs begin at the lower point of the lung, appearing like 

 small hydatids, and extend further and further into the abdomen, 

 in front of the kidneys. They are at first full of fluid. Soon after 

 the development of the abdominal air-sacs others are developed. 



The lungs are at first free, but afterward begin to be attached to 

 the ribs and to the spine. 



In the female embryo we first observe two oviducts, one on each 

 side of the basis, or stroma of the ovarium, which appears in a 

 relation to the primordial kidneys similar to that of the testes in the 

 male. At the period when the permanent kidneys have sent their 

 ureters to the cloaca, the oviducts have been developed as prolonga- 

 tions from that part, and, up to a certain point of development, they 

 are of equal size and length. Subsequently the left oviduct alone 

 continues to grow; the right remaining stationary or shrivels; 

 occasionally it may be discerned as a rudimentary in the mature 

 bird, but usually all trace of it has disappeared before hatching. 

 The left oviduct expands above or at its free end into the infundibular 

 orifice, where its parietes are very thin. As it descends, these in- 

 crease in thickness, and the efferent tube gradually acquires the 



