3. How to examine the eggs.—Have ready a basin of water, heated slightly above the temperature of the hand (i.e. to about 40° C.), and dissolve table-salt in it in the proportion of a level teaspoonful of salt to a pint of water. The young chicks will keep alive longer in this solution than in ordinary water. Tap the shell in the middle of its broad end, and open the air-chamber (a, [Fig. 185] ) completely. Then crack the shell in the middle of the length and, keeping the length of the egg horizontal, cut transversely round the middle of the shell with scissors in a vertical plane, until the halves are on the point of coming apart. Then lower the egg into the warm saline solution, pull the halves of the shell apart, and float out the contents. Examine the embryo carefully, making out as much as possible, and then snip round it with a pair of fine scissors to remove it from the yolk; float it into a watch-glass and cover it with weak alcohol (equal parts of water and spirits of wine). Examine it with a lens. After it has remained for a day in weak alcohol, put the embryo into strong alcohol in a small bottle (writing the age on a label), and preserve it.
Notice the gradual absorption of the white of the egg as development proceeds.
4. Chick after one day’s incubation.—Notice that the embryo is now to be distinguished as a streak crossing the germinal disc in a direction at right angles to the long axis of the egg. Notice a rounded swelling at one end of the embryo; this is the head. Place the egg before you with the broad end to your left, and observe that the head of the embryo points away from you.
5. Chick after two days’ incubation.—Observe the increase in size of the embryo; make a note of its length. The head and neck of the embryo are now almost covered by a very thin transparent bag which has grown over it from the sides. This bag is called the amnion; it is filled with fluid, and protects the embryo from jars. Remove the amnion and notice the large head; it is now twisted so that its left side lies against the yolk, while the rest of the embryo still lies “face-down.” Observe the large eye on the right side of the head; the left eye cannot be seen without turning the head over. Notice the heart, a small red dot which by help of a lens can be seen to beat rapidly. Surrounding the embryo is a circular network of blood-vessels which bring food from the yolk to the heart, to be distributed to the various parts of the body. How large is the circular area of blood-vessels?
6. Chick after three days’ incubation.—The white of the egg is distinctly shrunken, and the network of blood-vessels is much larger than before. Remove the amnion and notice the marked increase in size of the embryo, especially of the head. The right side of the head and neck are still turned towards the shell. They are now quite free from the yolk, but the body of the embryo communicates with the yolk by a short, wide tube, the yolk-stalk. Try to see a small pit, a little above and behind the large eye. This is the beginning of the right ear. Measure the embryo and the width of the surrounding network of blood-vessels. Watch, through a lens, the beating of the heart.
7. Chick after four days’ incubation.—Carefully cut open the amnion to see the embryo better. Observe that the young chick is still more completely folded off from the yolk, and that the yolk-stalk is consequently narrower than before. The head is so strongly bent upon itself that the snout almost touches the tail. The body also has now turned over so as to lie with its left side on the yolk. Observe the two pairs of small buds which are the rudiments of the limbs.
8. Chick after five days’ incubation.—Cut open the amnion, and notice the great increase in size of the embryo, and especially the enormous development of the head. The limbs now show signs of division into segments. Observe, under the hinder end of the body, a small, bladder-like outgrowth, the allantois, which in the later stages grows rapidly and spreads all round the inside of the shell. It is the breathing organ of the chick.
9. Effect of varnishing an egg.—Varnish an egg, and leave it under the hen with unvarnished eggs for the whole period of incubation (21 days). The varnished egg does not develop, because the varnish closes the pores of the shell and prevents the embryo from breathing.
