PRINCIPLES OF LIVESTOCK BREEDING. 6 
not differ greatly in the most widely different plants and animals. 
These cells are semifluid bits of living matter, each bounded by a 
membrane. Each contains within itself a differentiated portion 
called the nucleus. The details of the structure are brought out by 
the use of dyes, which are seized upon by certain cell structures and 
not by others. Thus if an animal of plant tissue is properly pre- 
served and stained with hematoxylin, a dye from logwood, a number 
of threadlike or rod-shaped bodies, called chromosomes, are made 
visible in the nucleus of each cell, through their taking on of a dark- 
blue color. 
It has been found that the number of these chromosomes in the 
cells of each kind of animal or plant is constant, with certain quali- 
fications, one of which will be taken up later. There are, for in- 
stance, 40 in swine, 48 in man, 8 in the fruit fly, 20 in corn, and 16 
in wheat. A great deal of attention has been devoted to the chromo- 
somes in recent years, as it has been demonstrated that they play an 
all-important part in heredity and the determination of sex/ We 
shall have occasion to refer to them later. 
A study of any rapidly growing part of a young animal soon re- 
veals cells which are in the act of dividing to form two cells. New 
cells are formed in the body only in this way. The individual begins 
his career as a single cell. This divides into two, the daughter cells 
divide, and so on until the trillions of cells of the adult body are 
produced. 
THE REPRODUCTIVE CELLS. 
The original single cell, though barely visible to the naked eye, 
must somehow contain within itself all the potentialities, physical 
and mental, of the organism into which it is to develop. The char- 
acteristics of both the paternal and maternal lines of ancestry must 
be represented in it. It is, in fact, the product of the fusion between 
two cells, one a sperm cell furnished by the male parent, and the 
other an ovum, or egg cell, from the female parent. 
The reproductive cells from the two sexes have very different appear- 
ances. In mammals, the ovum is a relatively large, spherical cell, 
just visible to the naked eye. In birds, the yolk of an egg is really a 
single ovum, distended to an enormous size by food material. The 
sperm cell is very much smaller and can be seen well only with a 
high-power microscope. It is something like a tadpole in shape, 
having a small cell body, containing little but the nucleus, and 
attached to this a long, whiplike process which beats rapidly while 
the cell is alive, enabling it to seek out and unite with the large, 
passive egg cell in the act of fertilization. Enormous numbers of 
sperm cells are produced by the male, but only one takes part in 
fertilization. After the first has penetrated the membrane of an 
