74 MANUAL OF HISTOLOGY. 



tact with surrounding substances. As to the composition of this layer, as 

 to its difference as compared to the softer protoplasm within, we know at 

 present nothing. Its power of resisting the action of reagents, such as 

 acids and alkalies, is for the most part very limited. 



Further metamorphosis of this superficial layer leads on through in- 

 termediate stages to the formation of the proper " cell wall." This 

 appears to possess a far greater power of resistance, in that the albu- 

 minous matter of the cortical layer has been converted into a substance, 

 which in its whole demeanour as regards various reagents, and in its im- 

 mutability, manifests a strong resemblance to, if not accordance with, 

 elastin. Even years ago it was asserted by Donders, that the membranes 

 of all animal cells consisted of elastin ; and although this expression of 

 that most excellent observer may be somewhat exaggerated, nevertheless 

 the capability of changing into a cell membrane possessed by the cortical 

 layer of protoplasm, gives support to the proposition that elastic matter 

 (elastin) may take its rise from the protein substances, although the 

 minutiae of the process of transition are not yet known. 



Passing on, finally, to the constitution of the nucleus, we have to dis- 

 tinguish between the envelope and contents of this originally vesicular 

 body. The contents, formed of a pellucid fluid, appear to be composed of 

 some soluble modification of protein matter ; for we can frequently pro- 

 duce a precipitate of small granules in it, by the action of alcohol, acids, 

 &c., as, for instance, in the nuclei of ganglion cells, and that of the 

 ovum. The envelope consists comparatively seldom of matter which 

 does not resist the action of acetic and other allied acids, as, for instance, 

 in the nuclei of the cells just mentioned. Usually and this is the 

 means for the recognition and distinguishing of the nucleus, long in use 

 empirically among histologists the envelope of the latter and the 

 remaining substance is not acted on by such acids. JSTow, although the 

 substances in question correspond in the last respect with the elastic 

 material of many cell-membranes, they yet differ from them most dis- 

 tinctly in their greater or less degree of solubility in alkalies. This has 

 been very properly pointed out by Kpllilter to be a distinguishing feature 

 between the nucleus and membrane of the cell. 



The chemical transformations which the nucleus undergoes during the 

 life of the cell are manifold, as, for instance, when it becomes solid, or 

 exchanges its vesicular nature for a granular one. The tendency, further, 

 of certain nuclei to deposit fats round about themselves is very striking, 

 a process which can go so far that, as is the case in certain cartilage 

 cells, finally, instead of a nucleus, nothing but a drop of oil can 

 be distinguished. It is also remarkable that pigments are seldom 

 seen in the nuclei of cells. Those of the epidermis of dark parts of 

 the skin, however, appear to be tinged by some brown colouring 

 matter. 



The nudeolus, owing to its minuteness, has almost completely escaped 

 chemical investigation hitherto. It is supposed, from its refracting pro- 

 perties, to consist of fat. 



Great uncertainty still prevails as to how far the products of the 

 decomposition of histogenic matters (already discussed in a former 

 section), which are found in the fluids saturating cellular tissue, are 

 originally constituents of the cell-body. It is also impossible to state, 

 even in the most favourable cases of simple cellular tissue, what products 

 of decomposition belong to the different parts of the cell, what to the 



