August 21, 1879] 



NATURE 



387 



psendopodia lay in seizing, in the manner of an Amaia, sucli 

 alimentary matter as may be found in the contents of the canal, 

 and applying it to the nutrition of the hydroid. 



What I had thus suggested with regard to Myriclhelahas been 

 since proved in certain planarian worms by MetschnikofT,' who 

 has seen the cells which line the alimentary canal in these ani- 

 mals act like independent Amceba, and engulph in their proto- 

 plasm such solid nutriment as may be contained in the canal. 

 When the planaria was fed with colouring matter these amoe- 

 boid cells became gorged with the coloured particles just as 

 would have happened in an Amccba when similarly fed. 



But it is not alone in such loosely aggregated cells as those of 

 the blood, or in the amoeboid cells of the alimentary canal, or in 

 such scattered constituents of the tissues as the pigment cells, or 

 in cells destined for an ultimate state of freedom, as the egg, 

 that there exists an independence. The whole complex organism 

 is a society of cells, in which every individual cell possesses an 

 independence, an autonomy, not at once so obvious as in the 

 blood cells, but not the less real. With this autonomy of each 

 element there is at the same time a subordination of each to the 

 whole, thus establishing a unity in the entire organism, and a 

 concert and harmony between all the phenomena of its life. 



In this society of cells each has its own work to perform, and 

 the life of the organism is made up of the lives of its component 

 cells. Here it is that we find most distinctly expressed the great 

 law of the physiological division of labour. In the lowest 

 organisms, where the whole being consists of a single cell, the 

 performance of all the processes which constitute its life must 

 devolve on the protoplasm of this one cell ; but as we pass to 

 more highly organised beings, the work becomes distributed 

 among a multitude of workers. These workers are the cells 

 which now make up the complex organism. The distribution of 

 labour, however, is not a uniform one, and we are not to sup- 

 pose that the work performed by each cell is but a repetition of 

 that of every other. For the life processes, which are accumu- 

 lated in the single cell of the unicellular organism become in the 

 more complex organism differentiated, some being intensified 

 and otherwise modified and allocated to special cells, or to special 

 groups of cells, which we call organs, and whose proper duty is 

 now to take charge of the special processes which have been 

 assigned to them. In all this we have a true division of labour 

 — a division of labour, however, by no means absolute ; for the 

 processes which are essential to the life of the cell must still 

 continue common to all the cells of the organism. No cell, 

 however great may be the differentiation of function in the 

 organism, can dispense with its irritability, the one constant and 

 essential property of every living cell. There thus devolves on 

 each cell or group of cells some special work which contributes 

 to the well-being of all, and their combined labours secure the 

 necessary conditions of life for every cell in the community, and 

 result in those complex and wonderful phenomena which con- 

 stitute the life of the higher organisms. 



We have hitherto considered the cell only as a mass of active 

 nucleated protoplasm, either absolutely naked, or partially in- 

 closed in a protective case, which still permits free contact of 

 the protoplasm with the surrounding medium. In very many 

 instances, however, the protoplasm becomes confined within re- 

 sisting walls, which entirely shut it in from all direct contact 

 with the medium which surrounds it. With the jilant this is 

 almost always so after the earliest stages of its life. Here the 

 protoplasm of the cells is endowed with the faculty of secreting 

 over its surface a firm, resisting membrane, composed of cellulose, 

 a substance destitute of nitrogen, thus totally different from the 

 contained protoplasm, and incapable of manifesting any of the 

 phenomena of life. 



Within the walls of cellulose the protoplasm is now closely 

 imprisoned, but we are not on that account to snpposc that it has 

 lost its activity, or has abandoned its work as a living being. 

 Though it is now no longer in direct contact with the surrounding 

 medium, it is not the less dependent on it, and the reaction 

 between the imprisoned protoplasm and the outer world is still 

 permitted by the permeability of the surrounding wall of 

 cellulose. 



When the protoplasm thus becomes turrounded by a cellulose 

 wall it seldom retains the uniform arrangement of its parts which 

 li often found in the naked cells. Minute cavities or vacuoles 

 make their appearance in it ; these increase in size ami run one 

 into the other, and may finally form one large cavity in the 



' " Ueber die Verdauungsorgane einiger Susswasser -TurbcUarien," 

 Zooiogischer Amei^ert ijecember, 1878. 



centre, which becomes filled with a watery fluid, known as the 

 Cell Sap. This condition of the cell was the first observed, and it 

 was it which suggested the often inapplicable term "cell." By 

 the formation of this central sap cavity the surrounding proto- 

 plasm is pushed aside, and pressed against the cellulose wall, 

 over which it now extends as a continuous layer. The nucleus 

 either continues near the centre, enveloped by a layer of proto- 

 plasm, which is connected by radiating bands of protoplasm with 

 that of the walls, or it accompanies the displaced protoplasm, 

 and lies embedded in this on the walls of the cell. 



We have abundant evidence to show that the imprisoned 

 protoplasm loses none of its activity. The Characa: constitute 

 an exceedingly interesting group of simple plants, common in 

 the clear water of ponds and of slowly running streams. The 

 cells of which they are built up are comparatively large, and, 

 like almost all vegetable cells, are each inclosed in a wall of 

 cellulose. The cellulose is perfectly transparent, and if the 

 microscope, even with a low power, be brought to bear on one 

 of these cells, a portion of its protoplasm 'may be seen in active 

 rotation, flowing up one side of the long tubular cell and down 

 the other, and sweeping on with it such more solid particles as 

 may become enveloped in its current. In another water plant, 

 the Valisneria spiralis, a similar active rotation of the protoplasm 

 may be seen in the cells of the leaf, where the continuous stream 

 of liquid protoplasm sweeping along the green granules of 

 chlorophyll, and even carrying the globular nucleus with it in its 

 current, presents one of the most beautiful of the many beautiful 

 phenomena which the microscope has revealed to us. 



In many other cells with large sap cavities, such as those which 

 form the stinging hairs of nettles and other kinds of vegetable 

 hairs, the protoplasmic lining of the wall may send off into the 

 sap cavity projecting ridges and strings, forming an irregular net- 

 work, along which, under a high power of the microscope, a 

 slow streaming of granules may be witnessed. The form and 

 position of this protoplasmic network undergo constant changes, 

 and the analogy with the changes of form in an Amccba becomes 

 obvious. The external wall of cellulose renders it impossible for 

 the confined protoplasm to emit, like a naked Ama-ba, pseudo- 

 podia from its outer side ; but on the inner side there is no 

 obstacle to the extension of the protoplasm, and here the cavity 

 of the cell becomes more or less completely traversed by proto- 

 plasmic projections from the wall. These often stretch them- 

 selves out in the form of thin filaments, which, meeting with a 

 neighbouring one, become fused into it ; they show currents of 

 granules streaming along their length, and after a time become 

 withdrawn and disappear. The vegetable cell, in short, with 

 its surrounding wall of cellulose, is in all essential points a closely 

 imprisoned Rhizopod. 



Further proof that the imprisoned protoplasm has lost by its 

 imprisonment none of its essential irritability, is afforded by the 

 fact that if the transp.arent cell of a Nitella, one of the simple 

 water plants just referred to, be touched under the microscope 

 with the point cf a blunt needle, its green protoplasm will be 

 seen to recede, under the irritation of the needle, from the 

 cellulose wall. If the cellulose wall of the comparatively large 

 cell which forms the 'entire plant in a Vaucheria, a unicellular 

 alga, very common in shallow ditches, be ruptured under the 

 microscope, its protoplasm will escape, and may then be often 

 seen to throw out pseudopodial projections and exhibit amoeboid 

 movements. 



]'".ven in the higher plants, w ithout adducing such obvious and 

 well-known instances as those of the Sensitive Plant and Venus's 

 Flytrap, the irritability of the protoplasm maybe easily rendered 

 manifest. There are many herbaceous plants in which if the 

 young succulent stem of a vigorously growing specimen receive 

 a sharp blow, of such a nature, however, as not to bruise its 

 tissues, or in any way wound it, the blow will sometimes be im- 

 mediately followed by a drooping of the stem commencing at 

 some distance above the point to which .the stroke bad been 

 applied ; its strength appears to have here suddenly left it, it is 

 no longer able to bear its own weight, and seems to be dying. 

 The protoplasm, however, of its cells, is in this instance not 

 killed, it is only stunned by the violence of the blow, and needs 

 time for its restoration. After remaining, it may be for some 

 hours, in this drooping and flaccid state, the stem begins to raise 

 itself, and soon regains its original vigour. This experiment 

 will generally succeed well in plants with a rather Large terminal 

 spike or raceme when the stroke is applied some little distance 

 below the inflorescence shortly before the expansion of the 

 flower. 



