IXCT'liATIOX OF lilRDS. 



29 



each end of the egg. Still move : a spiral tilamont runs through (^afh of these cords. 

 There are thus two fixed points on which the }'olk can rotate ; and the spiral filament, 

 surrounded by thickened albumen, is, by its greater weight, always inclined to the lowest 

 point. And, to perfect this beautiful arrangement, the elasticity of these spiral cords is 

 so nicely adapted to the force by which the yolk is borne up as to restrain it at a certain 

 point, and allow suificient space between the yolk and inner lining of the shell for the 

 white to lubricate the surface of the yollc, and thus to defend the embryo from injiuy. 



Various, indeed, are the situations in which tho mother-birds pass througli this 

 remarkable process. Oiu' chief knowledge of incubation arises, however, from the 

 attention that has been given to the domestic hen, and the results, therefore, we proceed 

 to notice. 



A considerable alteration takes place in the eggs a few hours after the hen has been 

 brooding over them. The form of tho embryo changes ; it acquires length, and its pro- 



FIG. 22 FORMATION OF THE VESSELS, AND DEVELOP- 

 MENT OF THE CHICKEN. 



FIG. 23.— THE EGG ON THE THIRD DAY. 



gress is seen by a blood-vessel issuing from either side, branching into numerous smaller 

 ones, which iinite at their termination, and become a boimdary on the covering of the yolk 

 (fig. 22). In the centre of this net-work of vessels is the embryo, and as it increases, so 

 do they midtiply, mitil they nearly pervade the membrane of the yolk. It appears that 

 when these vessels are first formed, and probably before, each branch is accompanied by 

 a vessel carrying yolk into the body of tho chick, that there may be a due supply for its 

 sustenance and growth. 



The vascular tissue which is to suffice for the primitive circulation, is a circidar net- work, 

 in the centre of which is a bent and palpitating vessel ; this vessel is the heart (fig. 23, a). 

 The whole net-work is bounded by the primigenjal vein (vp), v c is the caudal vein. 



The vascular net-work, the seat of this primitive circulation, may be regarded as a 

 lung which the embryo exhibits above it, and which is found placed in that part of the 

 egg where the oxygen can penetrate the most easily ; that is, below the air-chamber. 

 It is a simple pulmonary circulation, analogous to that of fishes, and lasts about two 

 daj's. Then begins a new circulation : the vascular net- work, deprived of its principal 

 venous trmiks, becomes almost mifit for the process of respiration, but its nutritive 

 functions are more developed. 



The yolk becomes flattened, and a portion of the white has penetrated the yolk-bag. 



