386 



NATURE 



\_August 21, 1879 



'Let us observe our Amaba a little closer. Like all living 

 beings, it must be nourished. It cannot trow as a crystal would 

 grow by accumulating on its surface molecule after molecule of 

 matter. It must feed. It must take into its substance the 

 necessary nutriment ; it must assimilate this nutriment, and 

 convert it into the material of which it is itself composed. 



If we seek, however, for a mouth by which the nutriment can 

 enter into its body, or a stomach by which this nutriment can be 

 digested, we seek in vain. Yet watch it for a moment as it lies 

 in a drop of water beneath our microscope. Some living denizen 

 of the same drop is in its neighbourhood, and its presence exerts 

 on the protoplasm of the Amccki a special stimulus which gives 

 rise to the movements necessary for the prehension of nutriment. 

 A stream of protoplasm instantly runs away from the body of 

 the Amaba towards the destined prey, envelops it in its current, 

 and then flows back with it to the central protoplasm, where it 

 sinks deeper and deeper into the soft yielding mass, and becomes 

 dissolved, digested, and assimilated in order that it may increase 

 the size and restore the energy of its captor. 



But again, like all living things, Amaba must multiply itself, 

 and so after attaining a certain size its nucleus divides into two 

 halves, and then the surrounding protoplasm becomes similarly 

 <:left, each half retaining one-half of the original nucleus. The 

 two new nucleated masses which thus arise now lead an indepen- 

 dent life, assimilate nutriment, and attain the size and characters 

 of the parent. 



We have just seen that in the body of an Amoeha we have the 

 type of a cell. Now both the fresh waters and the sea contain 

 many living beings beside Aviceba which never pass beyond the 

 condition of a simple cell. Many of these, instead of emitting 

 the broad lobe-like pseudopodia of Aviaba, have the faculty of 

 sending out long thin threads of protoplasm, which they can 

 again retract, and by the aid of which they capture their prey 

 or move from place to place. Simple structureless protoplasm 

 as they are, many of them fashion for themselves an outer 

 membranous or calcareous case, often of symmetrical form and 

 elaborate ornamentation, or construct a silicious skeleton of 

 radiating spicula, or crystal clear concentric spheres of exquisite 

 symmetry and beauty. 



Some move about by the aid of a flagellum, or long whip-like 

 projection of their bodies, by which they lash the surrounding 

 waters, and which, unlike the pseudopodia of Ansccba, cannot, 

 during active life, be withdrawn into the general protoplasm of 

 the body ; while among many others locomotion is eft^ected by 

 means of cilia — microscopic vibratible hairs, ^vhich are distributed 

 in various ways over the surface, and which, like the pseu- 

 dopodia aild flagella, are simple prolongations of their pro- 

 toplasm. 



In every one of these cases the entire body has the morpho- 

 logical value of a cell, and in this simple cell reside the whole 

 of the properties which mani'est themselves in the vital pheno- 

 .mena of the organism. 



The part fulfilled by these simple unicellular beings in the 

 economy of nature has at all times been very great, and many 

 geological formations, largely built up of their calcareous or 

 silicious skeletons, bear testimony to the multitudes in which 

 they must have swarmed in the waters of the ancient earth. 



Those which have thus come down to us from ancient times 

 owe their preservation to the presence of the hard persistent 

 structures secreted by their protoplasm, and must, after aU, have 

 formed but a very small proportion of the unicellular organisms 

 ■which peopled the ancient world, and there fulfilled the duties 

 allotted to them in nature, but whose soft, perishable bodies 

 have left no trace behind . 



In our own days similar unicellular organisms are at work, 

 taking their part silently and unobtrudvely in the greit scheme 

 of creation, and mostly destined, like their predecessors, 

 to leave behind them no record of their existence. The Red 

 Snow Plant, to which is mainly due the beautiful phenomenon 

 by which tracts of Arctic and Alpine snow become tinged of a 

 delicate crimson, is a microscopic organism whose whole body 

 consists of a simple spherical cell. In the protoplasm of this 

 little cell must reside all the essential attributes of life ; it must 

 grow by the reception of nutriment ; it must repeat by multipli- 

 cation that form which it has itself inherited from its parent ; 

 it must be able to respond to the stimulus of the physical con- 

 ditions by which it is surrounded. And there it is, with its 

 structure almost on the bounds of extremest simplification, 



wise regarded thari as an expression of an early differentiation in the structure 

 of the cell, and not, as has been maintained, an ultimate or " plastidular 

 ondition of protoplasm 



taking its allotted part in the economy of nature, combining 

 into living matter the lifeless elements which lie around it, 

 redeeming from sterility the regions of never-thawing ice, 

 and peopling with its countless millions the wastes of the 

 snow land.' 



But organisation does not long rest on this low stage of 

 unicellular simplicity, for as we pass from these lowest forms 

 into higher, we find cell added to cell, until many millions of 

 such units become associated in a single organism, where each 

 cell, or each group of cells, has its own special work, while all 

 combine for the welfare and unity of the whole. 



In the most complex animals, however, even in man himself, 

 the component cells, notwithstanding their frequent modification 

 and the lasual intimacy of their union, are far from losing their 

 individuality. Examine under the microscope a drop of blood 

 freshly taken from the human subject, or from any of the higher 

 animals. It is seen to be composed of a multitude of red 

 corpuscles, swimming in a nearly colourless liquid, and along 

 with these,-but in much smaller numbers, somewhat larger colour- 

 less corpuscles. The red corpuscles are modified cells, while the 

 colourless corpuscles are cells still retaining their typical form 

 and properties. These last are little masses of protoplasm, each 

 enveloping a central nucleus. Watch them. They will be seen 

 to change their shape ; they will project and withdraw pseudo- 

 podia, and creep about like an Amaba. But, more than this, 

 like an Amaba, they will take in solid matter as nutriment. 

 They may be fed with coloured food, which will then be seen 

 to have accumulated in the interior of their soft transparent pro- 

 toplasm ; and in some cases the colourless blood corpuscles have 

 actually been seen to devour their more diminutive companions, 

 the red ones. 



Again, there are certain cells filled with peculiar coloured 

 matters, and called pigment cells, which are especially abundant, 

 as constituents of the skin in fishes, frogs, and other low verte- 

 brate, as well as many invertebrate, animals. Under certain 

 stimuli, such as that of light, or of emotion, these pigment cells 

 change their form, protrude or retract pseudopodial prolongations 

 of their protoplasm, and assume the form of stars or of irregu- 

 larly lobed figures, or again draw themselves together into little 

 globular masses. To this change of form in the pigment cell the 

 rapid change of colour so frequently noticsd in the animals pro- 

 vided with them is to be attributed. 



The animal egg, which in its young state forms an element in 

 the structure of the parent organism, possesses in the relations 

 now under consideration a peculiar interest. The egg is a true 

 cell, consisting essentially of a lump of protoplasm inclosing a 

 nucleus, and having a nucleolus included in the interior of the 

 nucleus. While still very young it has no constant form and is 

 perpetually changing its shape. Indeed, it is often impossible to 

 distingiiish it from an Amaia ; and it may, like an Amaba, 

 wander from place to place by the aid of its pseudopodial pro- 

 jections. I have shown elsewhere = that the primitive egg of 

 the remarkable hydroid Myrioihcla, manifests amoeboid motions ; 

 while Haeckel has shown ' that in the sponges certain Amaba- 

 like organisms, which ate seen wandering about in the various 

 canals and cavities of their bodies, and had been until lately 

 regarded as parasites which had gained access from without, are 

 really the eggs of the sponge ; and a similar amoeboid condition 

 is presented by the very young eggs of even the highest animals. 

 Again, Reichenbach has proved* that during the develop- 

 ment of the crayfish the cells of the embryo throw out pseudo- 

 podia by which, exactly as in an Amaba, the yolk spheres which 

 serve as nutriment for the embryo are surrounded and engulphed 

 in the protoplasm of the cells. 



I had shown some years ago ' that in Myriothela, pseudo- 

 podial processes are being constantly projected from the walls 

 of the alimentary canal into its cavity. They appear as 

 direct extensions of a layer of clear, soft, homogeneous proto- 

 plasm \\ hich lies over the surface of the naked cells lining the 

 cavity, and which I now regard as the " Ilautschicht " or cortical 

 layer of these cells. I then suggested that the function of these 



' The Red Snow Plant {Trotococcus iiivalts) acts on the atmosphere 

 through the agency of chlorophyll, like the ordinary green plants. As in 

 these, chlorophyll is developed in it, .-ind is only withdrawn from view by the 

 predominant red pigment to which the Protococats owes one of its moit 

 striking characteristics. , 



= "On the Structure and Development of MyriotJula, Phil. Irans., 

 vol. clxv., 1875, p. 552. 



^ Jcnaisc/ie Zeiischr., iZji. t-i , 1. •• 



4 "Die Embryonanlage und erste Entw.ckelung des Flusskrebsc, 

 Zeitschr. f. ivissens. Zooiogie, 1877. 



5 Loc. cit. 



