14 ELEMENTARY STRUCTURE OF ORGANIZED BODIES. 



lular spaces disappear, as seen in figure 2, which represents 



the pith of the elder. They then 

 have the form of a honey-comb ; 

 whence they have derived their 

 name of cellules. 



38. All the organic tissues, whether 

 animal or vegetable, originate from 

 the cell. The cell is to the organ- 

 ized body what the primary form of the crystal is to the 

 secondary, in minerals. As a general fact, it may be stated 

 that animal cells, are smaller than vegetable a b 



cells, and contain a central dot or vesicle, called (S tjfe\ 

 nucleus. Hence those cells are called nucle- 

 ated cells (Fig. 3, a). Sometimes the nucleus Fig. 3. 

 itself contains a still smaller dot, called nucleolus (Z>). 



39. The elementary structure of vegetables is easily ob- 

 served in every part of a plant, and its cellular character 

 has been long known. But with the animal tissues there is 

 far greater difficulty. Their variations are so great, and 

 their transformations so diverse, that after the embryonic 

 period it is often difficult, even by the closest examination, 

 to detect their original structure. 



40. Several kinds of tissues have been designated in the 

 animal structure ; but their differences are not always well 

 marked, and they pass into each other by insensible shades. 

 Their modifications are still a matter of investigation, and 

 we refer only to the most important distinctions. 



41. The areolar tissue. It is the most generally diffused. 

 The cells are usually large, but irregular, with their walls 

 often imperfect. In man, as well as in the higher animals, 

 it is interposed, in layers of various thickness, between the 

 organs of the body, and contains more or less fat. Most of 

 the membranes are mere modifications of it. 



42. The cartilaginous tissue is composed of nucleated 



