THE DEVELOPMENT OF THE CHICK 



INTRODUCTION 



I. The Cell Theory 



The 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 hfe 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 spermatozoon 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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