STRUCTURE OF THE NUCLEUS. 179 



exhibits changes of form due to alterations of surface tension, which are comparable to the 

 amoeboid movements of living cells. Still more recently, Butschli has found that if oil is 

 rubbed up into a paste with certain alkaline salts in a moist condition, and some of the paste 

 is examined in water, the latter diffuses into the paste and converts it into a froth, which, 

 under the microscope, has an appearance not unlike the reticular part of protoplasm. 

 In such a froth streaming movements may be seen, lasting for a considerable time, and 

 changes of form may occur in the mass, due partly to continued diffusion through the soap- 

 like envelopes of the froth -bubbles or vacuoles, and partly to the bursting of these bubbles 

 when they become enlarged and approach the edge of the mass. Upon these observations 

 Butschli has based a theory that all protoplasm has such a vacuolated froth-like structure, 

 that the reticulum is only apparent, being the optical expression, of the material between the 

 vacuoles, and that the movements of protoplasm are produced by physical and chemical 

 processes analogous to those which cause the movements within the froth of oil and salt- 

 solution which he has employed. Whilst admitting the interest of Biitschli's observations, it 

 would, I think, be unwise to follow too far the deduction which he is inclined to draw from 

 them. For amongst other important objections which might be urged, the absence of 

 reticular appearance, and, therefore, of frothy structure from the active protoplasm of 

 pseudopodia is in itself fatal to the theory. This difficulty Butschli meets by assuming that 

 such structure is really there although the thinning out of the pseudopodia has rendered it 

 invisible. But the line of junction of the spongioplasm and hyaloplasm of an amoeboid cell 

 is sharp (fig. 205), and shows no tendency to fade off gradually into the pseudopodia, as on 

 Biitschli's assumption it should unquestionably do. 



Nageli, on theoretical grounds, has conceived that the essential living substance must in 

 its ultimate (ultra-microscopical) structure consist of solid particles (or systems of such 

 particles) surrounded by material of fluid consistence. To these hypothetical particles of 

 living matter (or to the systems which they form) he has given the name micellez (tagmata, 

 Pfeiffer), and he supposes that they may, like the substances known as ferments, produce 

 chemical changes in materials which are in contact with them without themselves undergoing 

 any permanent or perceptible change (catalytic action). 



THE NUCLEUS OP THE CELL. 



The nucleus is a minute vesicular body, placed generally near the centre of the 

 cell and embedded therefore in the protoplasm. In form it is round or oval in most 

 cases, but it may be elongated and folded or irregular in shape. Its size relatively 

 to that of the cell varies much in different instances, for sometimes there is so 

 small an amount of protoplasm that the nucleus appears to occupy nearly the whole 

 cell. This is the case with many of the cells which are met with in lymphatic 

 glands, and with the small nerve-cells which are found in the cerebellum and 

 elsewhere. On the other hand, the protoplasm of the cell, whether altered as in the 

 superficial layers of some stratified epithelia, or unaltered as in many of the white 

 corpuscles of the blood, may much exceed the nucleus in bulk. In absolute size, 

 the nucleus does not exhibit so considerable a variation as do the cells of the same 

 animal. There are, however, some notable exceptions ; thus the nucleus is abso- 

 lutely much larger in the ova and in many nerve-cells than it is in other cells of the 

 body. 



Structure of the nucleus. In the typical "resting" condition the nucleus is 

 always bounded by a well-defined wall, which encloses the nuclear contents. These 

 are of two kinds, formed and amorphous. To the latter the term nuclear fluid is 

 sometimes applied, and the former may be conveniently termed chromoplasm 

 (nucleoplasm or karyoplasm of authors) ; this term is used to include also the 

 substance which forms the wall of the nucleus. But it is by no means certain that 

 the homogeneous amorphous substance which occupies the interstices of the 

 chromoplasm is entirely fluid, so that it is better termed nuclear matrix. It is 

 very possible that this homogeneous matrix of the nucleus may be of the same 

 nature as the hyaloplasm of the cell-substance if indeed it is not actually continuous 

 with it, but the chromoplasm is not the same as the spongioplasm of the cell- 

 substance. 



