ELEMENTS OF STRUCTURE. 79 



years ago by the Dutch investigator Harting with the aid of the micro- 

 meter. 



A satisfactory physical analysis of cell-growth is not possible in >the 

 present state of science, and \ve can only here and there at most seize on 

 certain items of the process. 



If its surroundings afford the growing cell sufficient room for opera- 

 tion, and if those elements lying next to it are separated from it by con- 

 siderable intervals of soft, yielding matter, it increases uniformly in all 

 directions, and preserves its primary spherical form. But if, on tLe other 

 hand, growing cells are crowded closely together, owing to this increase 

 in size the various members of the crowd must come into contact 

 eventually, and consequently a mutual flattening of each individual 

 element ensue on account of their softness. It then depends, of course 

 merely upon mechanical moments, whether they will assume the flat- 

 tened sbape, and become squamous, or take on the elongated form. 



We meet, however, often enough with cells increasing in size, in tissues 

 of soft consistence, which are difficulties in the way of such an explana- 

 tion as that just given of the law of growth of cells, where the deposi- 

 tion of new molecules does not progress with uniformity, and in conse- 

 quence the cell becomes fusiform or pyriform, losing its original figure 

 altogether. If these additions to the substance of the cell take place 

 only at very limited points, they give rise not unfrequently to the forma- 

 tion of long processes in varying number. 



We cannot, however, hope to have attained much by this mode of 

 explanation of the shapes assumed by cells; for, just as the many species 

 of plants and animals possess each one its own special stamp, so do the 

 various kinds of cells of our body possess their own peculiar specific 

 characters whose origin mocks every kind of analysis we can apply. 



But not alone does the body of the cell grow, but the nucleus and 

 nucleolus also undergo an addition to their bulk, though in a minor 

 degree. The nucleus, on account of its similarity in nature to the cell, 

 may be supposed to increase in the same manner; and, in fact, we often 

 remark, besides the general enlargement, an irregular growth through 

 which me spheroidal body may become flat, elongated, and narrow, or 

 columnar, &c. The increase in size of the nucleolus is probably least of 

 all, although it can be distinguished in ganglion cells, and many others, 

 as, for instance, in the primitive ovum. 



In contrast to these cells there exist others in which, on account of the 

 growth or senescence of the body, the nucleus previously present disap- 

 pears is, in fact, dissolved. 



Thus the nuclei of the most superficial, or, in other words, the oldest 

 and largest cells of the epidermis vanish; and again the formative white 

 blood-corpuscle is endowed with a nucleus, which is absent in the red cell 

 later on, at least among human beings and mammals. 



Should the cell have developed around itself a more or less sharply 

 defined cortical layer of protoplasm, or an independent wall, this may 

 become increased in superficial extent by the deposit in it of new mole- 

 cules produced in the body of the cell. 



The envelopes, also, of growing cells frequently become thickened, 

 besides by a constant deposit of solid matter on their internal surfaces. 

 We shall have to take all these points into consideration below when con- 

 sidering cartilage cells. 



Other phenomena of growth which lead to a relinquishing of the 



