CHAPTER I. 

 DEVELOPMENT. 



The human body is composed of related structural 

 units that may be grouped in a series of gradually 

 increasing complexity. The simplest structural unit 

 is the cell, which is denned as a spacially limited 

 mass of protoplasm capable under certain conditions 

 of assimilation, growth, and reproduction. It is a 

 microscopic unit and forms the physical basis of life. 

 The next grade of units is a tissue, which consists of 

 a complex of similarly differentiated cells and their 

 derivatives. Embryonic tissues are mostly cellular, 

 while in some tissues of the adult body, such as bone 

 and cartilage, the cell products predominate. The 

 next higher grade of units is an organ, which struc- 

 turally consists of a complex of tissues forming a 

 body with a definite internal structure and external 

 form. Lastly, the highest structural unit is a system, 

 such as the nervous system, digestive system, respi- 

 ratory system, a series of which, collectively, make 

 up the human body. 



Histology is the branch of science that treats of 

 cells and their derivatives. These cells, in the adult, 

 are modified according to their intrinsic qualities, en- 

 vironment, function, and varied experiences, which 

 enable us to classify them and ultimately place them 

 in a few elementary groups. 



The ovum, or starting-point of every individual, 

 is a cell. Human embryology comprises its intra- 

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