THIS
HIS BULLETIN is written in simple language for

the man who breeds farm animals, who wants to learn the rudiments of the science of breeding and how to apply them in practice. It sets forth some of the known facts regarding the operation of the forces of heredity. Controversial subjects are avoided so far as possible.

The reader who cares to go deeper into the study of genetics is referred to textbooks on the subject, and particularly to Department Bulletin No. 905, “Principles of Livestock Breeding,” by Sewall Wright. It is published by the U. S. Department of Agriculture, and is obtainable from the Superintendent of Documents, Government Printing Office, Washington, D. C., for 15 cents.

Washington, D.C.

Issued November, 1920; revised June, 1924

IT

ESSENTIALS OF ANIMAL BREEDING.

BY GEORGE M. ROMMEL,
Formerly Chief of the Animal Husbandry Division, Bureau of Animal Industry.

N DEALING with the subject of heredity we must recognize

rule, have developed gradually and very slowly, by very small
changes, in short, by the process known as evolution. Second, what-
ever an animal has, so far as its inheritance is concerned, it gets from
its parents—they get theirs from their parents, and so on clear back
to the beginning; nothing was imposed from the outside. For the
present, accept this fact without question. We shall try to make it
clear later. Third, a very clear distinction should be drawn between
the effects of inheritance and the effects of food and environment.

Every one who is at all familiar with domestic animals knows what
a wonderful effect the food an animal eats has on its development,
especially when it is young. It is also known that the younger an
animal is the more readily it is influenced by its food. Now, from
the very first moment that a young animal begins to develop, from
the very instant that growth begins with the fertilization of the egg
cell, it receives food from some source or it dies. When it is only a
single cell it is being fed. If a young animal after birth is easily
influenced by its food, how much more will the delicate little
creature, known as the fetus or embryo, be influenced by the char-
acter of its food supply. A clear recognition of this fact will explain
many occurrences which are often attributed to supernatural in-
fluences, and, at the same time, the breeder who recognizes this fact
will not neglect proper attention to the nourishment and care of
the pregnant mother as well as the nursing animal.

Accidents and disease also are important in their effects on the
growing embryo. If an incubator is neglected on a cold night and
the eggs become chilled, the hatch is seriously impaired and a large

1 Mr. Rommel resigned from the department October 31, 1921.
NOTE.—In the preparation of this article, the writer received material assistance from
officers of the Bureau of Animal Industry, especially F. R. Marshall (now secretary of
the National Wool Growers' Association), Sewall Wright, Dr. John R. Mohler, and D. S.
Burch. He is also greatly indebted to DeWitt C. Wing, of Chicago, who made many help-
ful suggestions.

1

number of chicks are killed in the shell. If an in-foal mare is subjected to very heavy strains or forced to pull extremely heavy loads, the foal may be born too soon or even be killed in the womb. Contagious abortion may get into the herd without the owner's knowing it and cause the loss of a large proportion of calves. Heredity, however, has nothing whatever to do with any of these things.

THE SCIENCE OF BREEDING.

THE UNIT OF LIFE.

The smallest unit of life is the cell. A single cell is very small, usually of microscopic size. It is surrounded by a membrane, and each cell usually contains a nucleus—a sort of center of the things that go on in the cell. The animal body contains millions of cells, but those in which we are most interested at this moment are the cells from the union of which a new animal life begins. They are the germ cells. They contain the hereditary material which determines the identity of each individual and which is known as the germ plasm. This is the bit of life which passes down the line of descent with each succeeding generation. It appears in the mother in the egg cell, and in the father in the sperm cells. This material is composed of elements which seem to be handed on unchanged from generation to generation. Only a portion of the elements in the germ plasm of the parent goes into each egg or sperm, however, as is explained in more detail later. The element of chance enters to such an extent in determining which elements shall go into a particular germ cell that there may be a marked dissimilarity among individuals which are members of the same family. Even more important than the influence of food on the developing embryo and the young, growing animal is the operation of chance in determining the identity of the new life.

The cells from which all life starts are developed in the generative organs. The egg (the female cell) is developed in the ovary of the female; the sperm (the male cell) is developed in the testicle of the male. In mammals each female has two ovaries, one on each side, situated in the abdominal cavity. Connecting the ovaries with the uterus are the Fallopian tubes. The uterus is the womb, the chamber in which the young fetus develops until it is ready for birth. The uterus connects with the vagina, through which the sperm is introduced into the uterus and down which the young animal passes at birth. The genital organs and the urinary organs have the same outlets from the body.

When an egg matures in the ovary it passes down the Fallopian tubes into the uterus. If no sperm is introduced, the egg is thrown off infertile. In female animals an egg matures at fairly regular intervals, and this corresponds with some exactness to what is known as the period of heat or oestrum.

The period of heat varies considerably in different species and in different breeds and individuals of the same species. It usually occurs at intervals of about three weeks. In mares it appears on the ninth day after foaling, lasting two or three days. In cows it appears in from three to four weeks after calving when the cow is not suckling a calf, and in from six to eight weeks if she is. The