NORMAL HISTOLOGY. 



THE CELL 



ALL animals and plants are composed of minute structural elements 

 called "cells." With the exception of the low unicellular forms, in which 

 a single cell constitutes the entire organism, the fully developed animal com- 

 prises myriads of cells arranged as the tissues composing the various parts or 

 organs. 



Notwithstanding its complexity, the body of even the highest animal, 

 man, may be resolved into four elementary tissues epithelial, connective, 

 muscular and nervous which serve, primarily, for the purpose of protection, 

 connection and support, motion and control respectively. Every tissue con- 

 sists of two parts, the cells and the intercellular substance. Upon the first 

 of these, the cells, depend the vitality and growth of the tissue; while the 

 intercellular substance owes its production to the activity of the cells. 



The higher animals are derived from the union of the parent cells, 

 the spermatic filament and the ovum. This latter element, liberated from 

 the ovary of the mother, undergoes certain preparatory changes, known as 

 maturation, and then unites with the paternal germ-cell, the spermatic 

 filament or spermatozoon. The union of these two sex-cells results in 

 fertilization of the ovum. The fertilized ovum 

 immediately divides into the daughter cells, each 

 of which gives rise to two new elements; each 

 of these, in turn, produces two descendants, and so 

 on. As the result of segmentation, as this cycle of 

 repeated division is termed, a numerous progeny of 

 new cells arises from the original parent cells. The 



further division and differentiation of the segmenta- Exop'iasm Endopiasm 

 tion cells lead to the formation of the three germ-layers FlG I- _ Color i es? b iood- 

 the ectoderm, the mesoderm and the entoderm from corpuscle, representing type 



,.,,._. . , .. 11 of embryonal cell; cytoplasm 



which the definite embryo subsequently is evolved. exhibits differentiation into 

 Notwithstanding their diversity of form and size, |^P laBm and ex P lasm - > 

 as seen in the adult condition, the cells of the animal 



body possess certain features in common. So small that they can be seen 

 only when examined with the microscope, they consist of a minute mass of 

 gelatinous substance, the cell-body, in which lies embedded a still smaller round 

 or oval body, the nucleus. At times within the latter a distinct ^ dot, the 

 nucleolus, is seen. The original conception of the cell, as implied by its name, 

 was that of a minute sac, surrounded by a definite membrane or cell-wall, filled 

 with fluid and enclosing a second smaller sac, the nucleus, which, in turn, 

 contained a third saccule, the nucleolus. Subsequent study established the 

 gelatinous, not fluid, character of the substance of the cell-body, or cyto- 

 plasm, and the frequent absence of the cell-wall and the nucleolus. The 

 cell-body and the nucleus are, therefore, the only essential parts of such 

 structural units as are entitled to be regarded as true "cells" retaining and 



