January, 1913. 



KNOWLEDGE 



the latter enclosing as a rule one or more vacuoles 

 filled with fluid contents. This distinction into outer 

 and inner portions is not to be made in every cell, 

 since very young cells of embryonic tissues appear 

 during life to be almost entirely, if not quite, filled 

 with a mass of homogeneous living substance, in 

 which the nucleus and some granular structures are 

 suspended. 



Later observers, notably Nageli, Schwann, Flem- 

 ming, Cohnheim, Biitschli, Heidenhain, and many 

 others have attempted to explain the structure 

 brought out by " fixing " living cells rapidly by 

 means of such substances as osmic acid, chrom- 

 osmium acetic acid, alcohol-formalin-acetic acid, and 

 others, upon the lines that the appearances produced 

 by these reagents represent the true living structure 

 of the cell-protoplasm ; some of these appearances 

 are.no doubt, correct representations — as,forinstance, 

 the fixation of the vacuoles in the cytoplasm, and the 

 chromatin-network in the nucleus; but others are at 

 least of doubtful significance, and the present-day 

 cytologist hasto guard very carefullyagainst mistaking 

 an " artefact " in the cell for the true structure. One 

 conception as tocytoplasmic structure which, although 

 open to considerable criticism, is, nevertheless, pro- 

 visionally accepted by many workers at the subject, 

 is that which gives to it a reticular basis or 

 ground-substance of fibrils, the so-called spongioplasm, 

 in the meshes of which a more fluid portion, homo- 

 geneous in appearance, the hyaloplasm, exists; it 

 cannot be denied that even in the living cell, in some 

 instances, a fibrillated appearance is to be seen in 

 the cytoplasm, and this persists and becomes more 

 pronounced after fixation, t 



Another hypothesis which at one time appeared 

 likely to gain ground was that put forward by 

 Biitschli, who considered that cytoplasm might 

 possess an alveolar or froth-like structure, the spaces 

 in this being filled with liquid ; certainly the egg- 

 cells of certain animals, notably A scaris and Asterias, 

 possess a cytoplasm which suggests strongly the 

 alveolar structure, and cells of the nephridia of 

 Lumbricus and the Leech show a typical froth-like 

 appearance. But on the other hand there are many 

 cells the cytoplasm of which conforms more closely 

 to the reticular type ; and in the case of certain 

 large nerve-cells in the spinal cord and brain it has 

 been shown by Cajal and Bielchowski that a very 

 fine system of fibrils, the so-called neurofibrils, can 

 be brought into evidence by a process of treatment 

 with nitrate of silver and subsequent development. 

 These neurofibrils were also shown to extend into 

 the axis-cylinder process (axon) of the nerve-cell, as 

 well as into the processes known as dendrons. 



The granules seen in protoplasm are nowadays 

 considered to be very important structural elements, 



* It has been shewn that the Flagellata probably represent a sort of connecting link between the vegetable and the 



animal kingdoms. 



I See "The Vegetable Cell," by Hugo von Mohl, translated by Professor Henfrey. 1854. 



I Mention must be made, in this connection, of the experiments carried out by Hardy with colloids (such as various mixtures 



of gelatine and water) which pointed to the similarity of cytoplasm in physical constitution, to the " solid within a liquid " 



type of gelatine hydrosol, 



its own efforts to make use of the carbon dioxide 

 existing in the surrounding medium (atmosphere or 

 water), and in the majority of instances receives its 

 raw food materials in the form of complex carbo- 

 hydrate and protein compounds, the former of which 

 have already been built up for it by the vegetable 

 cell ; the latter, perhaps by the vegetable cell, but in 

 many cases by other animal organisms. The above 

 elementary physiological processes in a cell serve 

 to partially distinguish between the character of 

 the functions of vegetable and animal protoplasm, 

 although, as was pointed out, there may be no 

 structural distinction ; moreover, the explanation of 

 such variation in function must be looked for, 

 not in structural distinctions, but in the gradual 

 changes taking place during evolution from the 

 plant to the animal type,* changes, which, 

 although undoubtedly existent, still remain more 

 or less unexplained. 



With regard to the structure of the several parts 

 of the cell noted above, it will perhaps be best to 

 take these in order, and consider some of the hypo- 

 theses which have been advanced with respect to 

 their ultimate physical constitution. I do not pro- 

 pose to go into the consideration of the chemical 

 composition of protoplasm, as this somewhat 

 undecided question is one which is likely to lead 

 the reader too far afield, and, moreover, might have 

 the effect of confusing his mind as to the object 

 of this article, viz., the conception from a purely 

 biological point of view of cell-structure and function. 



(a) Structure of Cytoplasm. 



Early conjectures as to the internal structure of 

 cell-protoplasm were not of such a nature as to 

 enable much criticism, seeing that the methods then 

 in use for " fixing" cells and for staining their con- 

 stituent parts were hardly adequate to bring out 

 many structural details ; moreover, the microscope, 

 at the period when Hugo von Mohl made his inter- 

 esting discovery of the " primordial utricle " in 

 vegetable cells, was not the high-class instrument it 

 is nowadays. Nevertheless, many interesting and 

 accurate observations were made at that time, more 

 especially in connection with the composition and 

 structure of the cell-wall. t 



Protoplasm was at that period looked upon as 

 being quite homogeneous in physical constitution, 

 and it is only fair to state that there are many 

 nowadays who have come back to this view ; it was, 

 however, soon found that such a conception must be 

 modified, in that, in many living cells, notably Amoeba, 

 Aethalium, and numerous plant-cells, the main 

 cytoplasm was found to be made up of a clear outer 

 part, the ectoplasm, and an inner portion which was 

 granular and in some cases reticular, the endoplasm, 



