Development of a Mammal.--In mammals after fertilization the egg undergoes development within the body of the mother. Instead of blood vessels connecting the embryo with the yolk as in the chick, here the blood vessels are attached to an absorbing organ, known as the _placenta_. This structure sends branch-like processes into the wall of the _uterus_ (the organ which holds the embryo) and absorbs nourishment and oxygen by osmosis from the blood of the mother. After a length of time which varies in different species of mammals (from about three weeks in a guinea pig to twenty-two months in an elephant), the young animal is expelled by muscular contraction of the uterus, or is born. The young, usually, are born in a helpless condition, then nourished by milk furnished by the mother until they are able to take other food. Thus we see as we go higher in the scale of life fewer eggs formed, but those few eggs are more carefully protected and cared for by the parents. The chances of their growth into adults are much greater than in the cases when many eggs are produced. REFERENCE BOOKS ELEMENTARY Hunter, _Laboratory Problems in Civic Biology_. American Book Company. Bigelow, _Introduction to Biology_. The Macmillan Company. Cornell _Nature Study Leaflets_. Bulletins XVI, XVII. Davison, _Practical Zoology_, pages 185-199. American Book Company. Hodge, _Nature Study and Life_, Chaps. XVI, XVII. Ginn and Company. Sharpe, _Laboratory Manual_, pp. 195, 204-209. American Book Company. ADVANCED Dickerson, _The Frog Book_. Doubleday, Page and Company. Holmes, _The Biology of the Frog_. The Macmillan Company. Jordan, _Fishes_. Henry Holt and Company. Morgan, _The Development of the Frog's Egg_. The Macmillan Company. Needham, _General Biology_. Comstock Publishing Company. XVII. HEREDITY, VARIATION, PLANT AND ANIMAL BREEDING _Problems.--To determine what makes the offspring of animals or plants tend to be like their parents._ _To determine what makes the offspring of animals and plants differ from their parents._ _To learn about some methods of plant and animal breeding._ _(a) By selection._ _(b) By hybridizing._ _(c) By other methods._ _To learn about some methods of improving the human race._ _(a) By eugenics._ _(b) By euthenics._ SUGGESTIONS FOR LABORATORY WORK