5S 



THE BOOK OF POULTRY. 



eg'^ may contain a visible embryo. Again, since 

 even the new-laid egg is already an organism, 

 which has attained a certain stage of growth, it 

 is subject to disease, or weakness, or accident, 

 like other organisms. Thus an egg may be 

 fertile, and the germ may begin to develop, 

 but may perish at any stage from sheer lack of 



Fig. 24. — Chick on Second or Thin! Day. 



strength, precisely as a weakly baby may die 

 at any age. Quite apart from accidents or in- 

 juries whilst hatching, there is no doubt that in 

 many eggs the embryo is not strong enough in 

 itself ever to come to maturity. Such deaths 

 at various stages, within the shell, are in no 

 essential respect different from deaths of weakly 

 chickens at various early stages after leaving the 

 shell; the necessary vigour may fail the infant 

 creature at any particular time. So also the 

 embryo can be injured within the shell in various 

 ways ; and while it might be fanciful to say it 

 can be " frightened," there is much evidence to 

 show that it may suffer from some kind of 

 nervous shock, as in a severe thunderstorm. 



Whenever the Qg'g is again subjected to a 

 heat analogous to that of the hen's body, the 

 process of development is resumed, if the 

 interval has not been too long. There can 

 obviously be no definite limits to such an 

 interval. We have sent eggs to America which 

 hatched 60 per cent, after that voyage, and an 

 entire interval of thirty days ; and many sit- 

 tings have similarly hatched, after crossing the 

 Atlantic. On the other hand, in the earliest days 

 of artificial incubation it became notorious that 

 eggs laid more than four or five days, hatched 

 then rarely and with difficulty, proving that 

 after a very few days there was a change for the 

 worse in the vigour of the embryo. Yet again, 

 a hen allowed to steal her nest almost 

 always hatches well ; and it seems probable that 

 her periodical visits, with their short intervals of 

 warmth (for a hen at liberty rarely remains on 

 the nest more than an hour when laying), refresh 



Development 



during 

 Incubation. 



and re-invigorate the germ, and probably may 

 even carry on farther to some minute degree, 

 the process of development. 



It is needless to describe in detail the 

 development of the chick when steady incuba- 

 tion has been commenced. A few hours enlarge 

 the central pellucid spot, which 

 becomes oval, with a furrow down 

 the centre, and blood-vessels appear 

 round it ; then begins to develop 

 a double membrane called the amnion, whicla 

 at a later period entirely encloses the embryo 

 along with what is called the amniotic fluid. 

 By the second or third day the tiny embryo 

 enclosed in the amnion can be clearly seen as in 

 Fig. 24, surrounded by a patch upon the surface 

 of the yolk which is covered by fine blood-vessels. 

 The eyes can also be seen with a magnifying- 

 glass, as dark spots, and even the pulsation of 

 the heart. At or soon after the third day 

 another growth called the allantois begins to 

 push out from the digestive canal of the embryo, 

 between the two coats of the amnion, and at a 

 later period also encloses the embryo. By the 

 fifth or si.xth day the allantois may be observed 

 as a bag or sac protruding from the navel, 

 independent of the }'olk-sac (Fig. 25). By this 

 time rudiments of the wings and legs can be 

 clearly seen as buds or small clubs standing out 

 from the surface of the body, which has grown a 

 great deal. The network of blood-vessels has 

 also extended, and the yolk-sac is larger and 

 more defined. This and the developing allantois, 

 at about the seventh day, are more clearly shown 

 in Fig. 26, 



Fig. 25. — Fiflh Day: al, allantois. 



The allantois is, however, flattened and spread 

 out in reality between the outer and inner layers 

 of the amnion, where it gradually extends till it 

 entirely surrounds the growing chicken, close to 

 the outer shell and membrane of the (fgg. It is 

 furnished with a beautiful network of blood- 

 vessels, extended under the porous envelope 



