3. Pick off the pieces of shell and allow all the contents of the shell to flow out into a cup or deep saucer, taking care not to break the yolk.

a. State what has been done, and describe the position and color of the white and of the yolk of the egg.

b. Note two twisted strands extending from the yolk towards each end of the egg. These help to protect the yolk from sudden jars. Describe the position, appearance, and use of these strands.

4. Carefully turn the yolk until you notice a white spot. This spot is the beginning of an embryo chick.

Describe the position and appearance of a young chick embryo.

54. Reproduction and life history. In the preceding section we have seen that a bird's egg consists of a hard shell, a membrane, the white

and the yolk; and that on the outer surface of the latter is a tiny embryo. Let us now see how this egg is formed and developed.

In our study of seed-plants we learned that plant embryos are formed in the ovary

FIG. 53. Sperm-cells of various of a pistil after an egg-cell

animals.

has been fertilized by a sperm

(22, 27) and the fish (104)

cell. In the case of insects we find that egg-cells are produced in organs of the female known as ovaries and that before an egg-cell can develop into an embryo (except in rare cases) it must be fertilized by a sperm-cell (Fig. 53) which has been formed in the spermary of a male.

If the ovaries of a hen are examined, they will be found. to consist of a large number of spherical objects, the larger

ones being yellow, which vary in size from tiny dots to fullsized yolks (Fig. 54). If any one of these is examined carefully with a microscope, a single egg-cell may be found. After the yolk has attained its full size and the egg-cell has been

FIG. 54. Ovary of hen, and egg in egg-tube.

fertilized, it receives its coating of white, and the whole is covered with the membranes and the shell.

Immediately after fertilization takes place, by the process

of cell division many cells are formed. At the time the egg is laid, the chick embryo appears as a tiny white spot on the surface of the yolk when the egg is opened (53, 4). Further development of this embryo, however, cannot take place unless the egg is kept warm. This is brought about when

the hen broods over the eggs. Gradually the cells of the different external and internal organs are formed (Fig. 55) from the food material furnished by the yolk and the white of the egg, and at the end of three weeks the young chick breaks through the shell, and soon, under the protection of the mother hen, begins to search for food. When first hatched, the feathers are relatively small and downy. The further development of the chick is largely a matter of growth in size and of change in the character of the feathers.

55. Nests and care of young. The method of reproduction in all birds is much the same as that already described

FIG. 56. Comparative size of the eggs of ostrich, hen, and humming bird. (Photographed by E. R. Sanborn, N. Y. Zoological Park.)

for the chick. Many birds, however, are much more helpless when they emerge from the egg than are chickens, and so they are sheltered in nests, and the food of the young birds is brought to them by their parents until they are able to fly and for several days afterwards.

Nests differ greatly in their complexity and in the kind of material used. Some birds, for example the gulls and many other sea-birds, usually deposit their eggs on rocky ledges or in slight depressions in the sand along the shore. On the other hand, the Baltimore oriole constructs out of grasses, plant fibers, and strings a marvelous nest hanging high up in the trees, near the outer ends of branches (Fig. 73). Between these two extremes are all gradations of nest complexity. The eggs laid by birds vary in number, size, and color. The tiny humming bird, for

instance, lays two white eggs, each a third of an inch in diameter (Fig. 56); three to five greenish blue eggs, each nearly an inch in diameter, are usually found in a robin's nest, while an ostrich deposits twelve to fourteen eggs, each weighing three to four pounds.

56. Common methods of classification. One of the simplest ways of classifying birds is that of dividing them into groups according to the kind of food they eat. For instance, we may speak of fisheating, seed-eating, and insecteating birds. This, however, is far from being a scientific classification, since birds that differ considerably in structure, and therefore not closely related, frequently live upon the same kind of food. For example, both the pelican (Fig. 57)