THE CELL I I 



metrical. Haidenhain speaks of cells with such polarity as being dorso- 

 ventrally symmetrical. 



The nucleus cannot live outside of the cytoplasm. In general the 

 nucleus is found lying in the cell where it has the best opportunity for 

 the most extensive and intimate contact between its surface and the 

 main cytoplasmic mass. It usually lies in a central position in the cyto- 

 plasm, but it may lie at the extreme periphery. In fact it sometimes lies 

 so far out that it occupies a position outside of the general outline of the 

 cell, and is covered by a mere film of cytoplasm. In turn the cytoplasm 

 cannot live for long or reproduce itself without the nucleus. 



In plants the cell is usually inclosed within a cellulose cell-wall. In 

 animal cells there is, as a rule, but an indefinite cell-membrane; most of 

 the figures of animal cells will show this. In many tissues the cell-walls 

 or cell-membranes are not present. This results in a blending of the 

 cytoplasm so that the number of individual cells can only be determined 

 by the nuclei; the cell boundaries in such tissues cannot be determined. 

 Such a mass of cytoplasm, with frequent nuclei, is called a syncytium. 



One of the features of cell organization is size. With very rare excep- 

 tions the cell is a very small body only a few thousandths of a millimeter 

 in diameter, and while (relatively) some cells may be twice or ten times 

 as large as some others, yet they nearly always remain microscopic 

 bodies. In the few cases where they are macroscopic objects, as the 

 hen's ovum and certain low plants, we find that this unusual size is due, 

 not to a larger mass of protoplasm, 

 but to non-living contents or to 

 internal vacuoles or spaces. The 

 structure of protoplasm evidently 

 prohibits its working in more 

 than a certain sized mass, nor 

 is this an arbitrary rule. The 

 interchange of material between 

 cell mass and exterior, which is 



constantly and necessarily taking 



place in living protoplasm, would' 

 alone give us good grounds for 

 this conception, when we re- 

 member the fact that the surface, 



,, , , . , ... FIG. 12. .-'erve cell from steilate ganglion of 



through Which nutritive, waste, squid> Loligo Pealii _ ^ trophospongia or intra- 



and Other materials must pass, cellular blood channels; conn.t.fi., connective 



,, i ., tissue fibrils penetrating the cell, especially by 



increases as the square, while way O f trophoTpongia. 



the content or mass, which must 



be supplied, increases as the cube of the dimension. This idea is much 



strengthened when we remember that most of the very largest solid 



