DEVELOPMENT OF TISSUES AND ORGANS. 33 



cells, marked by a more or less exclusiveness of function, are termed 

 tissues. It must not be overlooked, however, that, though certain func- 

 tions are accentuated in individual tissues, they all possess remnants of 

 all the functions originally seen in undifferentiated protoplasm. Thus, 

 many besides the muscular tissues possess the power of contractility : 

 it is not the nervous system alone which retains the property of 

 responding to irritants. All the tissues are capable of reproducing them- 

 selves in part, and all possess the power of carrying on their own 

 nutritive processes if suitable food is supplied to them. 



We have seen that fecundation of the ovum leads to the develop- 

 ment of an immense number of new cells, and we have referred to the 

 modifications in form to which these segmentative spheres may be 

 subjected. 



Tissues are formed from these segmentative spheres in three different 

 ways (Wundt) : — 



1. Through formation of layers of cells. 



2. Through union of cells. 



3. Through excretions by cells. 



Often all of these methods are united in the formation of a compound 

 tissue which is functionally active as a unit. Such a tissue is called an 

 organ. The classification of tissues is based on anatomical grounds; of 

 organs, on physiological grounds. 



1. To the first group belong the epitheliums. In most of these the 

 only modifications which occur in the form of the cells are due to 

 mutual pressure from close contact. Such cells are therefore polygonal 

 or flattened, or, when growth is most marked in one axis, cylindrical. 

 The epitheliums, in series of layers or in single rows, cover the external 

 surfaces of the body as well as the coatings of the digestive, urinary, 

 genital and respiratory tracts which communicate with it, the ducts of 

 glands, and the closed serous sacs. In the latter locality they are called 

 endothelia, in the former epithelia. The hair and nails are modifications 

 of these tissues formed of small elongated cells grown together into 

 almost homogeneous tissues. The terminal portions of the organs of 

 special sense are also epithelial in nature. 



Epithelial cells are connected by a thin layer of an albuminous 

 cement substance, which during life is in a semi-fluid condition. 

 The shape of the cell may be columnar or squamous. Spindle-shaped and 

 club-shaped cells, as well as goblet cells, ciliated cells, epidermic cells, 

 and prickle cells, are all modifications of these shapes. Endothelial cells 

 are always of the squamous variety, arranged in single layers of flattened, 

 transparent cells with oval nuclei. When their form approaches the 

 columnar, as occasionally is the case in certain serous membranes, they 

 are then in an active state of division, are called germinating cells, and 



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