11. What peculiarities distinguish feathers which aid in flight from feathers which merely prevent loss of heat? (1906) 12. Describe the wing of a bird, both as to skeleton and as to external appearance. Compare the skeletal parts with those of the corresponding structures in a Mammal. (1908) [Pg 282] 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 [Pg 283] 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.