482 DEVELOPMENT OF THE CHICK. 



reptiles and mammals. It does not begin to function fully 

 until birth. 



In the chick the pronephros is, as would be expected, 

 reduced to a mere vestige. Its tubules occur in segments 

 5-15 (as reckoned by the somites), but are very imperfect. 

 They arise at the end of the second day and disappear on 

 the fourth day. The pronephric duct into which they open 

 arises partly by their union and partly by an outgrowth 

 from the anterior tubules which grows backwards until it 

 acquires an opening into the cloaca. This duct is the Wolffian 

 duct, and corresponds with the mesonephric duct of the 

 dog-fish. 



The mesonephros arises as a series of tubules in somites 

 16-32 (fig. 90, KC). Each tubule is a modified part of the 

 intermediate cell-mass, and opens into the Wolffian duct. 

 Each has, moreover, a Malpighian body at its commence- 

 ment. The whole organ receives its blood in part from the 

 dorsal aortal and in part from the posterior cardinal veins, 

 which thus form a renal portal system. Degeneration of the 

 mesonephros begins about the tenth day, and ultimately the 

 organ loses its excretory function and becomes converted 

 into the epididymis of the cock and into a functionless vestige 

 in the hen. 



The metanephros begins its development upon the fourth 

 day, and extends from somites 25-33. 



ee. The Organs of Sense. 



The Eye. As in vertebrates generally, the retina or 

 essential part of the eye is derived from the brain, whilst 

 the lens is an ingrowth of the surface epiblast. The retina 

 develops from the optic vesicle (figs. 91, 92). On the second 

 day these vesicles grow rapidly, and by the end of that day 

 are constricted at their base and pitted inwards on their outer 

 surface. The lens arises as a hollow pitting-in of the surface 

 epiblast, as in fig. 91, OL. 



On the third day the optic vesicle has doubled upon 

 itself to form the retina on the inner side and the pigment 

 layer on the outer. The stalk by which it is connected with 



