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substances that have been taken in, but not digested, are all forced out of the body; it becomes covered with a toughish skin, spherical and motionless, as shown at 1; then the inner portion of the sphere draws together, as shown at 2, into a smaller sphere a little within the one just described, and this inner sphere soon breaks up into very many exceedingly small ones seen at 3.
These gradually grow pear-shaped (4), and at length break out through the covering-skin, and swim away through the water by means of slender tails (shown at.5) spun out of their jelly-like substance at the smaller end. After about a day of this free-swimming life the slender tail is drawn in, and the animal becomes extremely like a Protamæeba (6, 7). But one more change needs here be followed: two or more of these Protamaba-like animals meet, melt into one, and that one sends out the long, interlacing false-feet shown in 12. A strange life-history, yet not so strange but that it is very closely followed by the plants of a humble group, which take in their food, and creep about, which even melt together into one, in the same marvellous way as the moneron just described.
1 The myxomycetes.
So indistinct is the partition between animals and plants, that the great German naturalist and microscopist Ehrenberg, the founder of the zoology and botany of the microscopical forms of life, was often misled in regard to the nature of his favorite objects of study. Although his researches lasted a third of the way into the present century, he was to the last so much in the dark in regard to the matter of a wellknown little organism (now recognized by all as a plant?), that he described it as having a mouth, an eye, and several stomachs.
It has seemed worth while to go into so much detail in regard to these lowest organisms, because they illustrate, better than any thing else can, the nature of the animal and the vegetable cell; and from some kind of a cell each animal and plant originates.
In the case of animals, the cell which is to grow into the fully developed individual is called the ovum, or egg; and generally it is not less simple in the highest than in the lowest animals, or, as Professor James Orton says,
" At the outset, all embryos, from the sponge to man, are indistinguishable from one another. They are mainly drops of fluid a little more transparent on one side than the other; and in all cases this almost homogeneous globule must develop three well-defined parts, — a germinal dot, germinal vesicle, and yolk."
1 Volvox globator.
Now, these parts correspond very closely with the nucleolus, the nucleus, and the protoplasınic
cell-contents of the ameba; and a figure drawn from one of these objects might readily be taken to represent the other. More than this, the egg in its earliest stages not only
looks like an amaba, but Fig. 12. - Mammalian Egg.
it also acts like one; for all eggs of animals in this earliest stage move about in the same sort of creeping way in which an amoeba moves.2
The same is true of the white corpuscles, or cells, found in the blood of all animals; and in the white blood of sea-snails the corpuscles have even been seen taking food, just as so many amæbas would.3
Now, just as the amoeba begins the process of self-division by halving its nucleus, so the egg, in every case, shows the beginning of the process of forming an embryo 4 by halving its germinative vesicle, and then the rest of the yolk. While this first halving of the yolk is going on, or as soon as it is finished, each of the newly formed portions is again divided into two, and so on till the whole contents of the egg is changed into a mulberry-like mass of little globular cells, much as the contents of the large, thick-walled sphere in the orange-colored moneron (Protomyxa) already described, became broken up into pear-shaped cells.
1 Comparative Zoology, p. 201. 2 Haeckel's Evolution of Man, i. p 134. 3 Ibid., p. 145. 4 The germ, or rudimentary animal, before birth.
Fig. 13. - Segmentation and Formation of the Mulberry Stage in Verte
Up to this point the process of development has been substantially the same for all animals:1 the story as told is equally true of an amoeba, a sea-urchin, a tape-worm, an oyster, or a frog.
But, while the close similarity in the process of development of all animals stops short at the period when the mulberry-like appearance has been reached, the sub-kingdoms above the pro
1 Except some of those very lowest forms which Haeckel ranks below protozoa, the “protista."