114 The Microscope. 



stance. The nucleus, inclosed by its delicate, always structureless,, 

 membrane, is of the same structure as the cell-body, though 

 possessing finer granules. The nucleolus and nucleoleolus are quite 

 like the nucleus, though more delicate. So much for the results of 

 Schwann's great work in the first half of the present century. 



Modern investigation has changed many of these older ideas, 

 though the features of the old cell are still recognized in the new. 

 At the present time the conception of a typical animal cell is about 

 like this: A cell-wall does not exist. This is without exception, 

 although in reaching this decision over- fine distinctions may hav© 

 been indulged. For all practical purposes the ovum with its zona 

 pellucida, and a few epithelial cells may be said to possess a limit- 

 ing membrane, but with these excepted there is little controversy 

 regarding its absence. The cell-body is composed of a net-work of 

 delicate fibrils, in the interstices of which is found a homogeneous, fluid 

 substance. In some cells this fluid holds pigment granules, fat, etc. 

 The nucleus is composed of a like, though more finely woven 

 net-work than that of the cell-body. The net-work may thicken at 

 the periphery and thus form a membrane for the nucleus; or a thin 

 membrane, independent of the intranuclear net-work, may be 

 present. The nucleolus is merely a thickening of the nuclear mesh. 

 This net- work may be continuous throughout the cell, that is, the 

 net- work of the cell-body may pass directly into that of the nucleus. 

 A few investigators have gone even further and claim that this net- 

 work is continued directly from cell to cell across the intercellular 

 substances. To demonstrate this net-work requires the aid of the 

 best immersion lenses and sometimes, unfortunately, the employ- 

 ment of special reagents. Notwithstanding, however, the careful 

 directions given by the gentlemen who have seen and studied this 

 inter- and intracellular mesh, it still remains for many of the best and 

 closest observers to see anything of it at all. 



Essential Paets of a Cell. — The earlier histologists believed 

 that every cell must possess the different parts of a typical cell, 

 though generally in a less perfect degree. That is, any cell would 

 be found to have a membrane, cell-body and nucleus. At the 

 present time nothing, but a cell-body is essential. A membrane 

 seldom or never exists, and though the majority of cells contain a 

 nucleus — a few having two or even three nuclei — a large number 

 contain no nucleus, though it is presumed that at some time they 

 must have had one. 



FoEMS AND Sizes of Cells. — The form of a typical cell is 

 spherical or spheroidal. This form, however, can be attained only 



