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Updated: May 11, 2025
As far back as 1850, Cohn wrote, apparently without any knowledge of what Payen had said before him: "The protoplasm of the botanist, and the contractile substance and sarcode of the zoologist, must be, if not identical, yet in a high degree analogous substances.
Some of the lowest organisms, in which no trace of nerves can be detected, are known to be sensible to light, owing, probably, to the presence in the sarcode of which they are mainly composed, of certain elements which, in organisms somewhat higher in the scale, become aggregated and developed into nerves specially endowed with the same sensibility.
Meantime, however, the students of animal histology were more and more impressed with the seeming preponderance of cell contents over cell walls in the tissues they studied. They, too, found the cell to be filled with a viscid, slimy fluid capable of motion. To this Dujardin gave the name of sarcode.
No definite arrangement and no approach to structure was observed in the sarcode, and no differentiation, with the exception of round bright-yellow oil-globules, very much like those found in some of the radiolarians, which are scattered, apparently irregularly, in the sarcode.
It was a condition chiefly confined to the caudal end, the sarcode having became diffluent, hyaline, and intensely rapid in the protrusion and retraction of its substance, while the nuclear body becomes enormously enlarged. These never appear alone; forms in a like condition are diffused throughout the fluid, and may swim in this state for hours.
How a nerve comes to be sensitive to light, hardly concerns us more than how life itself originated; but I may remark that, as some of the lowest organisms in which nerves cannot be detected, are capable of perceiving light, it does not seem impossible that certain sensitive elements in their sarcode should become aggregated and developed into nerves, endowed with this special sensibility.
It is comparatively huge in its proportions, its average extreme length being the 1/1000 of an inch. Its normal form is rigidly adhered to as that of a rotifer or a crustacean. Its body-substance is a structureless sarcode.
Darwin doubts not that it does slowly and by degrees by single acts, for instance, instead of by a succession of acts, aggregating into the semblance of an optic nerve certain elements in the sarcode of certain low organisms, spreading out the nerve thus formed into a network or retina, forming a number of separate pigment-cells into a homogeneous cornea, and following up these first steps by others which, how much soever more apparently complex, would cost comparatively little after the earlier and simpler ones had been taken.
Meanwhile, the diffluence causes a spreading and flattening of the sarcode and swimming gives place to creeping, while the flagella violently lash. In this condition two forms meet by apparent accident, the protrusions touch, and instant fusion supervenes.
This condition tensified, the amoeboid action quickened as here depicted, the agility of motion ceased, the nucleus body became strongly developed, and the whole sarcode was in a state of vivid and glittering action. If now it be sharply and specially looked for, it will be seen that the root of the flagella splits, dividing henceforth into two separate pairs.
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