Pseudopod

“ Among pseudopod forming organisms, the highest development of contractility is found in the foraminifera. As is well known, these organisms form finely anastomosing pseudopods which frequently cover the substratum with a network of protoplasmic strands. The terminal sections of these strands are frequently so thin and transparent that they cannot be seen easily with the microscope. As a rule the granular endoplasm is observable only in the main body of the organism and in the larger trunks of the pseudopods. ”

“ In fact as one watches the movements of a Biomyxa, for example, under moderately high magnification, one gains the impression that there seems to be no restriction imposed upon the extent of contractility in the pseudopods. They seem to possess perfect elasticity. As to the transformation of endoplasm into ectoplasm, little can be said, owing to the transparency of the protoplasm. But the whole of the pseudopod, when forming, seems to stream forward. As in Difflugia, the interior streams flow at about the same rate as the pseudopod as a whole advances. ”

“ There would seem to be a tendency therefore for it to accumulate at the end of a free pseudopod in such a form as discoides, and even under ordinary conditions of locomotion where there is occasional attachment{71} to the substratum by very short pseudopods, the surface layer is continually moving toward the anterior end on practically all sides. ”

“ Sometimes the whole of a pseudopod is thrown into a spiral, all of the turns being of equal size and only slightly motile. More than half of all the{128} pseudopods formed become spiralized at one time or another of their existence, the greater number of these being however relatively immotile. ”

“ If a still stronger stimulus is applied by firm pressure at the end of the pseudopod the process takes place with much greater violence. The differentiation of the protoplasm spreads centripetally from the point of stimulation over the whole pseudopod with great rapidity, and produces a quick retraction in the same, then involves the oppositely directed pseudopod, in which it then extends more and more slowly, until, proceeding in a centrifugal direction, it is at last gradually completely obliterated. ”

“ When new pseudopods are formed, or when old ones are retracted, and especially when both these phenomena occur at the same time and close together on a part of an older pseudopod, some of the details of coordination in streaming are readily made out. ”

“ The mass of endoplasm within a pseudopod moves practically always in one direction. In any cross-section of a pseudopod that is more or less cylindrical in shape, the endoplasm in the center moves most rapidly, that near to it less rapidly, while that near the ectoplasm moves very slowly. One never observes a forward stream on one side of the pseudopod and a backward stream on the other. Nor does one observe parallel streams of endoplasm flowing in opposite directions within the same{11} ”

“ Since a particle attached to the surface may remain for some time at the tip of a retracting pseudopod, while one that is attached to the sides of a pseudopod moves toward its base, it appears that the speed of the moving surface film is not directly correlated to the rate of transformation of ectoplasm into endoplasm. The slower speed of particles near the posterior end points also in this direction. ”

“ The body is usually of constant spherical form in relation to the floating habit. Nuclearia, however, shows amoeboid changes of general outline. The pseudopods are retractile, the axial filament being absorbed as the filament grows shorter and thicker and disappearing when the pseudopod merges into the ectoplasm, to be reformed at the same time with the pseudopod. ”

“ No recorded observations on the endoplasm along the length of a pseudopod can be arranged so as to form{88} a gradient which would suggest a similar gradient in metabolic rate; and if the endoplasm is a passively moved fluid as Hyman’s theory seems to imply, a metabolic gradient would seem to be precluded.
In the ectoplasm however there exists a time gradient; that at the base of a pseudopod is older than that near the tip, and observation generally tends to confirm the view that the older it is the firmer it becomes. ”

“ In this stage it possesses no main pseudopod as does proteus, discoides, laureata and other species, but there are three or four pseudopods extending actively in the general direction of locomotion. The physical characteristics of these pseudopods, in so far as streaming is affected, are different from those of the clavate amebas. The ectoplasmic tubes are relatively thicker, the endoplasm is less fluid, and new pseudopods are not formed so readily. It appears therefore that an increase of surface in the ameba serves to increase the amount of ectoplasm that is formed during locomotion. ”

“ It requires perhaps more energy for a pseudopod to flow off from the main stream at an angle of 120° than at an angle of 30°. But it is plain that as many pseudopods are withdrawn as are thrown out, and they are withdrawn at an angle against the main stream of endoplasm in the ameba that is the complement of the angle at which they were projected. Whatever energy might be saved therefore in the projection of a new pseudopod at a small angle with the main stream is lost in withdrawing the pseudopod against the stream at a correspondingly large angle. ”

“ Very young or small pseudopods in proteus have the same general appearance as the pseudopods of other large species (dubia, laureata, discoides, annulata, etc.) ; that is, there is a central axial stream of endoplasm surrounded by a layer of ectoplasm. But there is one difference even here, and that is the greater thickness of the ectoplasmic walls in proteus in proportion to the diameter of the pseudopod. The ectoplasmic tube however is not solid throughout, but is more or less honeycombed, somewhat like a network, with the spaces filled by endoplasm. ”

“ And as a corollary to this we may add that more pseudopods are formed, since ectoplasm can be formed more readily. (The verrucosa types possess very stiff ectoplasm, and they increase their surface by flattening out and by forming longitudinal ridges. They cannot for some unknown reason form pseudopods) . Again with the increase in the consistency of the protoplasm, the pseudopods become more slender (and stiffer) and more contractile, the most slender pseudopods (radiosa, flagellipodia) being very much more contractile than the larger ones of proteus or discoides, for example. ”

“ The pseudopods are usually straight, radiating from the central body. They possess usually a central axial rod of condensed or strongly gelatinized protoplasm around which is a layer of thick protoplasm with the properties of ectoplasm. Heliozoans for the most part move slowly; in fact many of them are pelagic and in these the power of locomotion on a solid substratum is very slow. There is however one species, Acanthocystis ludibunda, which, according to Penard (’04) , can move twenty times its diameter in one minute by rolling. ”

“ The animal part of the rhizopods is a gelatinous body called sarcode, a bit of scarcely organized protoplasm. Food is taken in through the outer wall, and is made into small pellets, which are surrounded by a digestive fluid in extemporized stomachs. Minute granules move about the interior, perhaps the substitute for a circulating fluid; and the outer wall can be moulded into the long processes called pseudopodia, used for locomotion and prehension. When these rhizopods secrete stony matter for a covering, the interior is the same structureless mass ”

“ For these species differ in the frequency with which pseudopods are formed, in their persistence when once formed, in the ratio of length to average diameter, in the general shape, in the frequency with which straight pseudopods are formed, in the speed of their formation{36} and withdrawal, in the manner of their withdrawal, in their disposition with respect to geometrical pattern, in the character of the bases of the pseudopods, in the form of the free ends, and so on. ”

“ That the specific character of the pseudopods, and the streaming which of course lies back of it, is not wholly or perhaps even largely, due to the specific structure of the protoplasm, is evident from a consideration of streaming in some other organisms, without a study of which, streaming in amebas can be only imperfectly understood. ”

“ They move in the same manner as Amœba radiosa. They draw in one pseudopod, stretch out another and float freely in the water in contrast to their limax state, in which they are always attached to some support. ”

“ This is the only instance known of a voice-organ in this situation, and moreover is the only case in all the Tenebrionidae in which any sound-producing organ has been discovered. The larvae exhibit but little variety, they are elongate and cylindrical, with harder integument than is usual in Coleopterous larvae; they have six thoracic legs, and at the under side of the posterior extremity the anus serves as a very short pseudopod. The resemblance of these larvae to those of Elateridae is considerable ”

“ The most noticeable feature about the Myxosporidian trophozoite is its amoeboid and Rhizopod-like character. Pseudopodia of various kinds, from long slender ones (fig. 3, B) to short blunt lobose ones, are of general occurrence, being most easily observed, of Morphology. course, in the free-living forms. The pseudopodia serve chiefly for movement and attachment, and never, it should be noted, for the injection of solid food-particles, as in the case of Amoebae. The general protoplasm is divisible into ectoplasm and endoplasm. ”

“ There are large amœbæ, which thrust out strongly mobile feet from their [130] unicellular body, broad, flaplike processes of the naked cell body which constantly change their form, size, and place. If they are killed and examined with the aid of the best methods of coloring, it is quite impossible to detect any structure in them; and this is also true of the pseudopodia of the mycetozoa and many other rhizopods. Moreover, the slow flowing movement of the fluid protoplasm shows clearly that there cannot be any composition out of fine fixed elements in the body. ”

“ They are characterized by the radiate pseudopods, finely tapering at the apex, springing abruptly from the superficial protoplasm, containing a denser, rather permanent axial rod (figs. 1 (1) , 2 (2) ) ; protoplasm without a clear ectoplasm or pellicle, often frothy with large vacuoles, like the alveoli of Radiolaria; nucleus 1 or numerous; skeleton absent, gelatinous or of separate siliceous fibres, plates or spicules, rarely complete and latticed; reproduction by simple fission or by brood-formation, often syngamous; form usually nearly spherical, rarely changing slowly. ”

“ They are never fixed, but in almost continual movement, due to differences of moisture, warmth, light, or chemical action. If, for instance, a moist body is brought into contact with one of their projections, or "pseudopods," the protoplasm seems to roll itself in that direction, and so the whole organism gradually changes its place. ”

“ Most of them are without legs, but a good many have three pairs of small legs, and there are numerous cases in which the surface of the body is furnished above or below with swellings believed to act as pseudopods (Fig. 84) , and help the larvae to move about in their galleries; but this is probably not the sole function of these organs, as their surface is varied in character, and often not of a kind that appears specially adapted to assist in locomotion. ”

“ Pseudopods usually granular, fine flexible, tapering generally, not freely branching; reproducing sometimes by simple fission, but more frequently by multiple fission in a brood-cyst whose walls may be multiple. Plasmodium formation occasional, but never leading to the formation of a massive fructification: other syngamic processes unknown, and probably non-existent. ”

“ Their shell is either thin or extended many times by long slender tapering spines, and the protoplasm outside has the same character as that of the Radiolaria (q.v.) , being differentiated into jelly containing enormous vacuoles and traversed by reticulate strands of granular protoplasm. These coalesce into a peripheral zone from which protrude the pseudopods, here rather radiate than reticulate. Most genera and most species are cosmopolitan ”

“ The spine-like processes are very thin and short. (Schaudinn '95.) The marine form found at Woods Hole probably belongs to this species, as described by Schaudinn. The short pseudopodia which give to the periphery a fringed appearance are quite regularly placed in connection with the pseudopodia. The latter are not so long as twice the body diameter, the longest being not more than equal to the diameter of the sphere. The body inside of the gelatinous covering is thickly coated with bright yellow cells similar to those on Radiolaria. ”

“ We are now therefore in a position to say that amebas are definitely and thoroughly organized; that they are not really “shapeless”; that they are not more subject to variation than a higher organism is; and that each species differs from all others in probably every visible detail. The large variety of pseudopods observed in different species are seen not to be the result of physical or extrinsic chemical forces acting upon ectoplasms differing in some mere physical character as viscosity. ”