Epithelial Structures. 



also in its general activity or metabolism. It contains a characteristic 

 formed material, chromatin, and frequently also a minute spherical body, 

 the nucleolus. The chief features of a typical cell are illustrated in the 

 accompanying figure (1) of the developing ovum, the latter being a 

 single cell, noteworthy for its large size, and also one in,which the 

 external form is not greatly modified, as it is in the majority of the cells 

 of the body. Its enclosing membrane, the zona pellucida, by which in 

 its natural position in the ovary it is separated from the surrounding 

 follicular cells, is considered to belong in part to the latter. 



As fundamental living matter, protoplasm possesses certain properties 

 on which the functions of the body ultimately depend. Considered 

 collectively, these functions are not so well illustrated in the higher or 

 multicellular organisms, in which particular functions are assigned to 

 particular cells, as in the lower unicellular organisms, in which all func- 

 tions are discharged by a single cell. In simple or protozoan animals 

 the protoplasm is seen to be capable of ingesting food-materials, of 

 discharging waste, of changing its 

 form, and of reacting in one way or b.{ 



another to stimuli arising outside of 

 the body. Moreover, the protozoan 

 cell is capable of giving rise to new 

 cells by division of its substance into 

 two parts, which process originates in 

 the nucleus, and is associated at some 

 stage, usually at least, with union or 

 conjugation of parent cells. 



All the cells of the body of a multi- 

 cellular organism are products of a 

 single cell, the fertilized egg, but the 

 latter is a product of fusion of two 

 primary elements, the spermatozoon 

 of the male parent and the ovum 

 of the female. The fertilized egg 

 does not exhibit the functions of a 

 one-celled body, but possesses the 

 potential of these functions, and the latter appear, to a large extent 

 individually, in the differentiation of its division products into special- 

 ized tissue-elements. 



The primary tissues of the body are of four kinds — epithelial, con- 

 nective, muscular, and nervous. To these — the fixed tissues — are to be 

 added the fluid substances, blood and lymph, in which the cell elements, 

 the red and white corpuscles, or in the latter case the white elements 

 alone, are suspended in a fluid medium. 



Fig. 1. Developing ovum of the rabbit. 

 From a section of the ovary: chr., chromatin; 

 n.m., nuclear membrane; p.f., cells of the 

 primary ovarian follicle ; z. p., zona pellucida; 

 pr., protoplasm. 



1. Epithelial Tissues. 



Epithelial tissues are distinguished chiefly as surface investments, 

 such as those of the exterior of the body, the interior of the alimentary 

 canal, the lungs, the respiratory and accessory respiratory tracts, and the 



