PHYSIOLOGICAL 373 



is that a high degree of stability may be attained by turgidity, 

 which practically means the intracellular pressure on the cell- 

 walls. The stem of the common Mimulus is mostly water, but it 

 serves to raise the golden flowers far out of the ditch. This is because 

 of the turgidity of the cells; and the same is true of the cells com- 

 posing the lips of the jelly-fish or the notochord of the lancelet. 



The second half of the answer is that protoplasm continually tends 

 to make for itself a upporting framework. In the simplest cases the 

 living matter secrete ' a non-living material that serves as a sort of 

 scaffolding for further growth. There is a precipitation of non-living 

 waste materials; in a few cases by-products of everyday activity 

 may be utilised; in other cases the protoplasm may die away; in 

 one way or another protoplasm may make for itself a supporting 

 framework. 



The other difficulty concerns the division of labour within the 

 cell, which may be usefully compared to a one-roomed house, with 

 a multitude of activities all occurring at once within a limited 

 radius. Why do they not interfere with one another? The modern 

 answer to this question is simply that protoplasm is a film- pervaded 

 or film-partitioned system. The living cell is partitioned with 

 extremely delicate films, not usually demonstrable in any direct 

 way, which have diffusion-hindering properties, and thus allow 

 dissimilar chemical processes to occur in contiguity. So we have 

 exchanged the visible intricacy apparent to earlier histologists, but 

 very largely artefact, for an intricacy that is usually invisible, yet 

 seems to be real. 



The Nucleus. — ^The microtechnical methods referred to in 

 connection with protoplasm have also been utilised in the study of 

 the nucleus, which was discovered by Robert Brown, whom Hum- 

 boldt called "facile princeps botanicorum". Here, however, the 

 picture presented by fixed and stained specimens seems to corre- 

 spond a little more closely to reality. In a normal resting nucleus 

 there is a fine skein of a material called "linin", and on this there 

 lie scattered granules of a protein substance which stains deeply 

 and is therefore called chromatin. The nucleus also contains a 

 compact body called a nucleolus, or there may be more than one. 

 In some cases a nucleolus seems to be a reserve of chromatin ; in 

 other cases it is more like a globule of waste material; in general, a 

 nucleolus is an inconstant and variable portion of the nucleus. 

 Bathing the chromatin there is a complex nuclear sap or karyo- 

 lymph, and surrounding the whole there is a nuclear membrane — 

 a "semi-permeable" membrane which lets certain substances, but 

 not others, in and out, in the give and take between nucleoplasm 

 and cytoplasm. 



Such is the resting nucleus of a cell, but in the ordinary process of 

 cell-division (the mode known as "karyokinesis" or "mitosis"), the 



