DEVELOPEMENT. 
71 
bag. After the sixth clay the allantois grows with in¬ 
credible rapidity until it has reached the inner wall of 
the egg; here it applies itself, and now brings about the 
chemical exchanges between the in- and outside. The 
veins leading from the heart into it branch out into 
so-called capillary vessels, which do not prevent the 
giving off of the carbon and absorption of fresh oxygen 
in the blood circulating through them. All interchange 
takes place perfectly and easily, notwithstanding the 
apparent obstacles arising from that peculiar organic 
action of the membranes, percolation or exudation 
(enclosmosis and exomosis), which, though proven without 
doubt, still remain but indifferently explained. In this 
manner the chicken begins to breathe in the egg, and 
thereby introduces into the body the action of animal 
heat. Through this it becomes strengthened and is 
enabled to dispense for some time with warmth from 
outside : thus it is already in some way independent. 
From this time on, till the bird creeps out of the shell, 
the further course of development is really only the 
enlargement of those parts of the body which have been 
planned during the previous days. The protuberance at 
the fore end of the head (sinciput) divides and lengthens, 
but can only be recognised as the beak by the tenth day; 
the legs and wings, which before were exactly alike, 
stretch and extend themselves, taking, almost at the 
same time with the beak, their individual formation; 
the lungs grow uninterruptedly until the last day of 
incubation, by which time they have arrived at their 
perfect state. After the fifteenth day the feathers, in the 
case of the domestic Fowl, begin to sprout from the 
skin; with other birds, at the corresponding date, some 
streaks of down at least. By about the twentieth day, in 
