CHAPTER XVI. THE DEVELOPMENT AND EDUCATION OF THE CHICK.

52. A HEN’S EGG.

Obtain three or four hen’s-eggs, and make the following observations upon them:

1. =External appearance.=—What is the _colour_ of the egg? What is its _shape_? How does the shape differ from that of a cricket ball or other sphere? The shape of an egg is said to be ovoid. [What is the difference between an ovoid and an oval?] Measure, with a tape, the length and breadth of the egg, and also its distance round, (i) in the direction of its greatest length, (ii) in the direction of its greatest width. What advantages has this shape? Could a hen sit so comfortably upon her eggs if they had sharp corners? Put an egg upon the table and roll it gently. Does it roll in a straight line? Does it roll far before coming to rest? Compare the rolling of a cricket ball or other sphere. Is it an advantage that an egg is not likely to roll far, or in a straight line? Why?

2. =The strength of the shell.=—Hold an egg with your thumb at one end and your first or second finger at the other, and press exactly in the line of the length of the egg. You cannot break the shell.

3. =Structure.=—(_a_) _The shell._—Tap the egg gently at the middle of its broad end until the shell cracks. Then carefully remove small pieces of the shell and notice the _shell-membrane_, a tough skin which is closely applied to the inside of the shell. Snip through the membrane in the middle of the broad end; notice the _air-chamber_ which lies beneath it. Observe the inner membrane which separates the air-chamber from the inside of the egg. Hold a piece of shell up to the light, and notice the small, almost transparent dots. The shell is perforated by very small pores, through which the air can pass.

(_b_) _The white of the egg._—Tap the shell so as to crack it all round at its widest part; raise bits of shell carefully and see the membrane here. Tear through the membrane and notice that in this region there is no air-space, but the white lies just beneath the shell-membrane. Separate the halves of the shell, notice the position and shape of the yolk, and then let the contents of the egg fall gently into a basin. Observe the appearance, colour, and transparency of the white, and try to distinguish two tangled cords of firmer white—the _balancers_ (Fig. 185)—arising close to the yellow yolk.

(_c_) _The yolk._—What is the shape of the yolk as it lies in the basin? How does it differ from the shape of a yolk suspended naturally in the white? What is the cause of the change of shape? Notice carefully a small paler patch in the middle of the upper surface. This is the lightest part of the yolk, so that the yolk always settles with this part uppermost after any turning of the egg, and therefore the pale patch is always more directly exposed to the heat of the hen’s body (during incubation) than is any other part of the yolk. Prick the yolk and notice that the yellow, fluid contents flow out. You have evidently pierced the thin bag which formerly preserved the shape.

(_d_) _A hard-boiled egg._—Boil an egg in water for five minutes, and then chip round the shell in the direction of the length and, with a sharp knife, cut the whole egg into halves along this plane. Make a drawing of the section, indicating the shell, shell-membrane, air-chamber, white, and yolk in position. Observe that the white is no longer transparent and fluid, but an opaque, white, and elastic solid. Try to peel off the white in layers. They will probably break off short, but you may be able to see that the white is deposited in spiral sheets around the yolk.

=The hen’s egg.=—A hen’s egg bears a somewhat similar relation to the adult bird that the maize or bean seed (Chapter I.) bears to the adult plant; for the egg contains

(1) a speck of living matter which little by little grows and becomes marked off, by orderly arrangement, to form the various regions and organs of the adult animal; and

(2) a store of food, which enables the young chick to develop in security, without being hampered, whilst still weak and helpless, by the necessity of earning its own living. The changes by which the speck of living matter becomes the perfect chick will be considered in the next section. We must now examine the structure of the egg itself, and see what provision it contains for the nourishment and safety of the developing bird.

The egg is ovoid in =shape=, one end being distinctly broader than the other. This shape has the advantage of preventing the egg from rolling very far when placed upon a slightly inclined surface, and it is worthy of notice that the eggs of birds which lay on cliffs and other exposed situations are usually more elongated and pointed than others, so that when stirred they do not roll away, but simply describe a small circle and come to rest again. In the case of eggs which are laid in cup-shaped nests (Fig. 197) the pear-shape lends itself to close packing, and thus allows the eggs to be more easily kept warm by the parent bird.

The =shell= (_sh._, Fig. 185) is composed of a chalky material, and is perforated by small pores, through which the developing chick obtains fresh oxygen from the air, and gives off its surplus carbon dioxide (p. 242). A small piece of shell easily snaps, but the shape of the complete shell so distributes an outside pressure, especially one in the direction of the long axis, that relatively great force is required to break it. The shell is lined by a thin, parchment-like =membrane= (_sh. m._). At the broad end of the egg this membrane is double, and the two layers enclose an =air-chamber= (_a_).

Inside the shell-membrane are the white or albumin, and the yolk. The =white= (_alb´._) is a viscous, transparent fluid. Its innermost part (_alb._), which immediately surrounds the yolk, is of thicker consistency than the rest, and is prolonged into two twisted cords called the =balancers= (_ch._), which suspend the yolk in position.

The =yolk= is a golden-yellow fluid enclosed in a thin, elastic membrane and hence preserving a spherical shape. On its upper surface (but under the membrane) is a small circular patch (_bl._) of paler colour. This patch, called the =germinal disc=, is about ⅛” in diameter; it contains the living matter from which the chick will be formed; the rest of the yolk, and the white, are simply a store of inert food which is used up by the growing chick.

=The work of the balancers.=—In order that the small living patch of pale yolk, the germinal disc, may grow and develop into the chick it must be kept warm. When eggs are hatched in the natural manner, the heat is supplied by the body of the sitting hen, and the upper part of the egg is consequently kept warmer than the rest. It is important that the living part of the egg shall always lie nearest the hen’s body and thus be kept warm, and this is secured by an arrangement as effective as it is simple. The yolk is lightest in the neighbourhood of the germinal disc, and therefore always lies with this part uppermost. If the egg is slowly turned over, the yolk remains “right side up.” If it is turned over quickly, the yolk soon swings round into its original position. The twisted cords of white, the balancers, not only sling up the yolk and guard it from being thrown to one side by a sudden movement, but they also prevent it from rotating too quickly—and so possibly injuring the delicate body of the young chick—when it rights itself after the egg has been turned over.

The balancers are rendered necessary by the hen’s habit of turning her eggs two or three times a day. It is often supposed that the eggs are turned in order to keep them equally warm on all sides, but this is unnecessary. Most probably the egg is turned in order to alter slightly the young chick’s position from time to time, and allow its parts to grow naturally, unimpeded by the other contents of the egg.

53. THE DEVELOPMENT OF THE CHICK.

1. =A simple incubator.=—Eggs are best incubated in the natural manner, that is, by the warmth of the hen’s body; but if a sitting hen cannot be obtained, an ordinary water-oven, such as is used in chemical laboratories, may be made to answer. It should be heated by a self-regulating burner, and kept at a temperature of about 40° C. The eggs should be turned two or three times a day, and the air of the oven should be kept moist by sprinkling water upon pieces of cloth, blotting paper, or hay, kept with the eggs. Spring is the most favourable time of the year for making the observations, as eggs laid at other seasons are not always in a condition to produce chicks.

2. =How to mark the eggs.=—The most instructive changes take place during the first five days of incubation. If all the stages of the first five days are to form the subject of one lesson, an egg should be marked “5” with pencil, and then put into the incubator or under the hen five days before the lesson; a day later, an egg numbered “4” should be put in, and so on. The numbers will then indicate the length of incubation at the time of the lesson, and the eggs should be examined in order, from 1 to 5. If one egg is to be examined each day, five should be put in the incubator at the same time; no numbering will then be required.

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.

=The early development of the chick.=—When the hen’s egg is exposed to a temperature of about 40°C., the =germinal disc=—the circular patch of living matter which lies, somewhat like a small inverted watch-glass, upon the upper surface of the yolk—grows larger, and its parts are gradually modified to form the various regions of the chick’s body. The white and yolk of the egg are used up during the process, and by the time they are absorbed, the young bird is in a condition to break the shell and take up the activities of an independent life. To watch the orderly appearance and gradual development of the different systems of organs is most fascinating, and fortunately the observation of the main features presents no great difficulty if the foregoing instructions are followed.

The first signs of the chick are to be seen (Fig. 186), towards the end of the =first day= of incubation, in a streak which crosses the germinal disc in a direction at right angles to the long axis of the egg. One end of the streak is distinctly rounded, and this is the =head= end (_hd._). In almost all cases, if the egg be placed so that its broad end is to the observer’s left, the head of the embryo will be directed away from him.

=A day later=, the embryo is markedly larger, and is partly covered by a double membrane called the =amnion=.[17] Surrounding the embryo is now (Fig. 188) a network of fine blood-vessels which ramify over the yolk and carry small particles of yolk to feed the growing organs. At the end of the second day the network could be about covered by a sixpenny piece. On each side, its veins join to form a single small tube running to the =heart= (_ht._), a tiny red dot which is situated behind the head and can be seen, by the help of a lens, to be beating rapidly. The head has grown more than the rest of the embryo, owing to the rapid development of the primitive brain, and is bent forwards. The head has turned over a little, so that its right side is directed towards the egg-shell; the rest of the embryo still lies “face-down.” The right =eye= (_e._) can easily be seen as a dark spot on the side of the head.

By the end of the =third day= (Fig. 189) the head and neck are raised distinctly above the yolk, and the tail is also more plainly marked off, so that the body is connected with the yolk only by a short wide tube, the =yolk-stalk= (_yk.st._, Fig. 187, _B_). The neck as well as the head has now turned so as to lie with the right side directed towards the shell. The commencement of the =ear= is to be seen on each side as a small pit (_au._) a little above and behind the large eye.

On the =fourth day= the embryo turns to lie entirely on its left side. It becomes more completely folded off from the yolk, and the connecting yolk-stalk is a narrower tube than before. The previously formed organs increase in size and complexity. The disproportionate size of the head, owing to the great development of the brain, is still more marked than before, and the head is so strongly bent upon itself that the snout almost touches the tail. One of the most noteworthy features of the fourth day is the appearance of the =limbs=. As yet these are merely a pair of small buds (_fl._, _hl._, Fig. 190), and no joints can be detected in them.

By the end of the =fifth day= (Fig. 191) the head is enormous, and the limbs now show signs of being divided into definite segments. A thin bladder-like structure—the =allantois= (_all._), which appeared on the fourth day—has grown out from the lower part of the body, behind the yolk-stalk. Its mode of origin is well shown in Fig. 192. It rapidly increases in size, and soon extends over the embryo (Figs. 192 _B_, and 191) and becomes closely applied to the shell-membrane. Air passes through the pores of the shell, and its oxygen is taken up by the blood which circulates in the vessels of the allantois. At the same time, waste carbon dioxide is able to escape from the blood to the outer air. The allantois is therefore the =breathing organ= of the developing chick. If the egg is varnished and the shell thus rendered air-tight, the embryo dies of suffocation at an early stage.

The chief organs of the bird are now established, and the later development may be sketched more briefly. By the end of the =ninth day= (Fig. 193) the white of the egg is almost used up; the yolk, however, is still large, and is connected with the chick’s body by the narrow yolk-stalk. It thus appears that the white is not directly absorbed by the chick, but is first taken up by the yolk and afterwards passed on by the yolk blood-vessels which run to the heart. By this time, too, the allantois has spread at least halfway round the inside of the shell, that a supply of oxygen adequate to the increased needs of the animal may be obtained from the air. The chick has now a characteristic bird-like appearance; the beak has appeared; feathers have begun to sprout; the neck is long and slender; and the segments of the limbs, including the fingers and toes, are well defined.

About the =fourteenth day= the chick turns so as to lie lengthwise in the shell, with its head near the broad end. The yolk-sac dwindles in size, and at last, about the twentieth day, it is drawn into the interior of the body. Now the chick becomes restless, and—usually on the =twenty-first day=—thrusts its beak through the inner shell-membrane into the air-chamber at the broad end of the egg. For the first time it draws refreshing air into its lungs, and is stimulated to break the shell by a knob on its beak, and to creep out into the world.

54. THE EDUCATION AND LATER GROWTH OF THE CHICK.[18]

1. =Hatching.=—On the 20th or 21st day of incubation remove three or four eggs from the clutch under the hen, and keep them in a warm cosy place so that you may watch the process of hatching. Can you hear the chick tapping the inside of the shell? Could it have been _taught_ to tap? Which part of the shell cracks first? When the shell has cracked, can you hear the chick chirping? Could it have been taught to chirp? Imitate the chirp and listen for a response. Keep the newly-hatched chicks in a warm, soft place. How soon do they recover from the exhaustion of hatching? Do the chicks show a liking for warm corners? Are they afraid of being touched gently?

2. =Locomotion.=—How soon do the chicks begin to run about? Do they stumble upon obstacles, or do they avoid or leap over them? Do they use their wings in running, or in jumping down a small step? Put a chick into a basket, and lower the basket quickly through the air, being careful not to let the chick fall out. Does it move its wings? How?

3. =Feeding and drinking.=—Lay a few grains of soaked wheat upon the ground before a chick which has not yet been fed. Does it peck at them instinctively? Tap a grain with the point of a pencil; does the chick now peck at it? Does it hit the grain at the first try? Does it ever strike at a grain which is out of reach? Watch a hen with her chicks; does she teach them to peck at food? How? Do the chicks know instinctively the difference between food and grains of sand, or have they to learn? Try if a chick can distinguish between a small worm and a bit of red worsted. Are young chicks afraid of large insects?

Take a chick, which has not yet drunk, to a small pool (a few drops) of water. Does it seem to know the use of the water? Induce it to peck at the water; does it now drink? How?

4. =The crouching-instinct.=—Clap your hands suddenly and loudly near young chicks. How do the chicks behave? Does this behaviour render them less conspicuous?

5. =Non-recognition of hen.=—Having made the above observations on chicks hatched apart from the hen, put the chicks among their brothers and sisters with the parent hen. Do the new-comers respond, as readily as the others, to the clucking, etc., of the hen?

6. =Maternal training and protection.=—How many different meanings can you recognise in the different sounds of the hen’s voice? How does she call and protect the chicks when any danger threatens them?

7. =The voice of the chick.=—Notice the different sounds by which a chick expresses pleasure, alarm, distress, etc.

8. =Preening and scratching.=—At what age do chickens begin to preen their feathers, and to scratch the ground? Try to discover if these are instinctive activities, or whether they have to be learnt from the hen.

9. =Change in plumage.=—At what age are the down feathers replaced by the true plumage? At what age is it possible to distinguish the young cocks from the young hens?

10. =External characters, etc., of adults.=—Notice the flowing tail-feathers, hackles (the long feathers on the neck and loins), comb, and spurs of the cock-birds, and their absence or small size in the hens. What differences are there as regards voice? How does the hen announce that she has laid an egg? Which sex is the more pugnacious?

=The recently hatched chick.=—The newly-hatched chick is clothed with fine down-feathers (pp. 273 and 277). It is generally exhausted by its struggles to escape from the shell, but it soon recovers, and is able to run about freely on the second day after hatching. Young birds which, like the chick, are active immediately after hatching, are said to be =precocious=.

=Instinct and education.=—From the first, the chick performs certain movements which are obviously =instinctive=, that is, which have not been acquired by any process of imitation or instruction. Even in the egg, it may be heard chirping soon after it has taken its first breath of air; and the complex activities of walking, running, jumping over obstacles, and, later, preening the feathers and scratching the ground—each of which involves the nicest adjustment of several muscles—are also instinctive. Certain other powers have to be learnt; the hen, for example, lifts and drops before the chick a particle of food which she wishes it to seize, and it soon learns to peck. At first its pecks generally fall a little short of the objects, but presently it becomes very adroit at catching food. “A chick a day or so old will catch a running fly at from the seventh to the twelfth shot.”[19] There does not appear to be much instinctive recognition of the difference between objects which are valuable as food and those which are the reverse. Such discrimination comes only by experience, and to a very young chick a piece of red worsted and a small worm are equally attractive before being seized. In the same way, the nature of water has to be learnt. Chicks peck at drops of water, but do not seem to know the use of water until their beaks are wetted, when they drink instinctively in the usual manner.

The use of the wings is also a matter of instinct, but it is acquired somewhat late, and is assisted by parental encouragement and example even in birds which are noted for their powers of flight. Fowls do not fly much, but chicks may be observed to use their wings as an aid in running or jumping. Prof. Lloyd Morgan[20] quotes an interesting experiment which shows well how deeply the reliance on flight is stamped in bird-nature. “If a chick a day or two old be placed in a basket, held firmly in the hand, and then lowered rapidly through the air, the fledgling will stretch out his little immature wings in such an attitude as would make them break the fall were they fully developed; or will, if he be a little older, flap them with flight-like action, in either case showing an instinctive response.”

=Hens and chickens.=—Chickens which have been hatched in the natural manner are, from the first, under the control of the hen (Fig. 194), and her anxious care for their welfare has always been considered one of the most beautiful examples of the maternal instinct. In their defence her courage is unbounded; should she discover a succulent worm or other dainty morsel, her ordinary complacent cluck changes to a note of invitation, to which the brood at once responds; and at her danger-signal the chickens run to take refuge under her wings, or crouch motionless against the ground. The chirp of a chick also is capable of expressing several states of feeling, such as contentment, pleasure, alarm, and distress. These different notes are not only perfectly intelligible to the hen, but are familiar to anyone who has had experience of poultry.

In the face of these well-known instances of the perfect understanding between the hen and her chickens, it is somewhat surprising to find that chickens, hatched apart from the hen and then placed with the rest of the brood after a few days, at first pay very little attention to the hen, except for their instinctive tendency to nestle into warm places, and do not seem to understand her clucks.

=Adult fowls.=—The down-feathers are soon shed and replaced by the true plumage, and the chicken gradually takes on the appearance of the adult. A marked difference is generally to be seen between the cocks and hens (Fig. 195) of any breed of poultry. The cock is usually provided with flowing tail-feathers, hackles (elongated feathers on the neck and loins), a prominent comb, and “spurs”—features which are either absent or much less perfectly developed in the hens. As great differences exist between the voices of the two sexes, the crow by which the pugnacious male challenges his rivals, and the cackle of the hen when she has laid an egg, being familiar examples.

EXERCISES ON CHAPTER XVI.

1. Point out the differences in appearance and position between the white and yolk of a fowl’s egg. What becomes of each during the development of the egg?

2. When does the heart of a chick begin to beat? Describe its appearance at this time, and explain what must be done to expose it to view.

3. Explain exactly where the chick is to be found in a fertile egg which has been incubated for three days. How big is a chick of that age? Of what colour is it? Does it give any signs of life? (1898)

4. What is the amnion? Where, in a developing chick, is it to be found, what is its appearance, when and how is it formed, and what is its use?

5. Explain the nature and use of the “balancers” of an egg.

6. Explain how a developing chick breathes.

7. What new organs are first seen distinctly on the fourth and fifth days respectively of incubation of a hen’s egg? Explain exactly how you would expose them to view, and describe their appearance.

8. What is the period of incubation of a duck’s egg? Observe and describe exactly how the hatching duckling breaks open the shell and escapes from it.

9. Observe and describe a duckling’s first attempt to swim. Is the action instinctive or not?

10. Make other observations upon ducklings, comparing them in as many respects as possible with chicks, and write careful accounts of the results you obtain.

11. How does a fowl’s egg which has been hatched for four days differ from a fresh-laid egg? Describe the new structures which have formed in it, so far as they can be made out by the unaided eye. (1904)

12. Where does the chick begin to form in the egg? Explain the arrangement which brings it as near as possible to the body of the sitting hen. (1905)

13. What is the use of the cloudy masses attached to opposite sides of the yolk of a fowl’s egg? (1906)

FOOTNOTES:

[17] The amnion originates, early in the second day, as a double fold of the yolk-surface in front of the embryo. Similar folds arise round the sides and tail, forming a low wall (Fig. 187, _A_); the folds gradually grow over the embryo (Fig. 187, _B_) until, during the fourth day, they meet (Fig. 187, _C_) and enclose it in a protective transparent bag, containing a watery fluid.

[18] I desire to acknowledge my indebtedness, in drawing up this section, to Prof. Lloyd Morgan’s _Habit and Instinct_ (Arnold).

[19] Lloyd Morgan’s _Habit and Instinct_ (Edward Arnold).

[20] Lloyd Morgan’s _Habit and Instinct_ (Edward Arnold).