344 PERSONAL AND CIVIC HYGIENE 



Conclusions. 1. Should you think there would likely be more or 

 fewer eggs if they are not only fertilized within the body of the mother, 

 but also hatched there ? 



2. Explain whether better care is shown when eggs are deposited in 

 the water to be fertilized and hatched, or when such processes take place 

 within the body of the mother. 



8. Which should be the larger because of contained food, eggs that 

 are laid, or those that are hatched within the body of the mother ? 

 Note. When any young are hatched within the body of the mother, 

 they soon use up the food yolk of the egg, and thereupon develop a 

 special set of blood tubes connecting with those of the mother, through 

 which they get their food supply. 



2. BIRDS 



Observations. 1. Break a hen's egg into a saucer. Note a small cir- 

 cular spot somewhat different from the yolk in color. Note, This is 

 the so-called germinal layer, and locates the real tiny egg. The yolk and 

 the white constitute food for the growing embryo. 



2. Has the embryo evidently developed much by the time the egg was 

 laid? 



3. Since the fertilized egg cells of birds are deposited outside of the 

 body, can you see any reason for a shell ? 



4. Why would it not be well that birds lay their eggs before the eggs 

 are fertilized ? Do such animals as fishes and frogs lay eggs with shells ? 

 Where are the eggs laid, and in what numbers ? 



5. Compare the ovary of a bird with that of a flower. What likenesses 

 do you find ? What differences ? 



Conclusions. 1. What are the main differences in the process of 

 fertilization of a bird's egg and the egg in the ovule of the lily ? Note. 

 The sperm cell of the male bird is very minute and is provided with a lash 

 to whip itself along. 



2. Why are eggs necessarily larger than sperm ? Which most readily 

 admit of independent movement, sperm or egg ? Note. We here find a 

 fine example of physiological division of labor ; on the one hand con- 

 jugating cells must be very motile and consequently minute, so that they 

 may meet in such a liquid as water, where they are commonly liberated 

 (examples : water plants, ferns and mosses, frogs and most fishes, etc.); 

 and on the other hand, they must be so supplied with food that the young 

 embryo may be provided for until it is able to shift for itself. These two 

 requirements are here evidently met by a physiological division of labor 

 between the two conjugating cells. One cell, called for convenience the 



