THE DEVELOPMENT OF THE CHICK 
INTRODUCTION 
J. Tuer CeLtt THEORY 
Tue fundamental basis of the general conceptions of embry- 
ology, as of other biological disciplines, is the cell theory. The 
organism is composed of innumerable vital units, the cells, each 
of which has its independent life. The life of the organism as a 
whole is a product of the combined activity of all the cells. New 
cells arise always by subdivision of pre-existing cells, and new 
generations of the organism from liberated cells of the parental 
body. The protozoa, however, have the grade of organization 
of single cells, and the daughter-cells arising by fission constitute 
at the same time new generations. In some metazoa new gen- 
erations may arise asexually by a process of budding, as in Hydra, 
or of fission, as in some Turbellaria; such cases constitute excep- 
tions to the rule that new generations arise from liberated cells 
of the parental body, but the rule holds without exception for 
all cases of sexual reproduction. 
The body consists of various functional parts or organs; each 
of these again consists of various tissues, and the tissues are com- 
posed of specific kinds of cells. The reproductive organs, or 
gonads, are characterized by the production of germ-cells, ova 
in the female gonad or ovary, and spermatozoa in the male gonad 
or testis. However large the ovum may be, and in the hen it 
is the part of the egg known as the yolk, it is, nevertheless, a 
single cell at the time that it leaves the ovary in all animals. 
Similarly the spermatozo6n is a single cell. An ovum and sper- 
matozoon unite, in the manner to be described later, and con- 
stitute a single cell by fusion, the fertilized ovum or oosperm. 
This cell divides and forms two; each of the daughter-cells divides, 
making four, and the number of cells steadily increases by suc- 
cessive divisions of all daughter-cells, so that a large number 
of cells is rapidly produced. Organs are formed by successive 
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