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correspondence from the fact already stated that the embryonic development of the individual is a brief recapitulation of the ancestral development of the species or larger group. The egg of the lowest vertebrate, amphioxus, shows these changes in a simple and apparently primitive form.
The fertilized egg of any animal consists of a single cell, a little mass of protoplasm containing a nucleus and surrounded by a structureless membrane. The egg is globular. The nucleus undergoes certain very peculiar, still but little understood, changes and divides into two. The protoplasm also soon divides into two masses clustering each around its own nucleus. The plane of division will be marked around the outside by a circular furrow,
but the cells will still remain united by a large part of the membrane which bounds their adjacent, newly formed, internal faces.
Let us suppose that the egg lay so that the first plane of division was vertical and extending north and south. Each cell or half of the egg will divide into two precisely as before. The new plane of division will be vertical, but extending east and west. Each plane passes through the centre of the egg, and the four cells are of the same form and size, like muchrounded quarters of an orange. The third plane will lie horizontal or equatorial, and will divide each of
these quarters into an upper and lower octant. The cells keep on dividing rapidly, the eight form sixteen, then thirty-two, etc. The sharp angle by which the cells met at the centre has become rounded off, and has left a little space, the segmentation cavity, filled with fluid in the middle of the embryo. The cells continue to press or be crowded away from the centre and form a layer one cell deep on the surface of the sphere. This embryo, resembling a hollow rubber ball filled with fluid, is called a blastosphere. It corresponds in structure with the fully developed volvox, except, of course, in lacking reproductive cells.
GASTRULA. HATSCHEK, FROM
Outer layer is the ectoderm ; inner layer, the entoderm; internal cavity, the archenteron; mouth of cavity, blastopore.
If the rubber ball has a hole in it so that I can squeeze out the water, I can thrust the one-half into the other, and change the ball into a double-walled cup. A similar change takes place in the embryo. The cells of the lower half of the blastosphere are slightly larger than those of the upper half. This lower hemisphere flattens and then thrusts itself, or is invaginated, into the upper hemisphere of smaller cells and forms its lining. This cup-shaped embryo is called the gastrula. The cup deepens somewhat and becomes ovoid. Take a boiled egg, make a hole in the smaller end and remove the yolk, and you have a passable model of a gastrula. The shell corresponds to the ectoderm or outer layer of smaller cells; the
layer of "white" represents the entoderm or lining of larger cells. The space occupied by the yolk corresponds to the archenteron or primitive digestive cavity; and the opening at the end to the primitive mouth or blastopore. Ectoderm and entoderm unite around the mouth. Both the blastosphere and gastrula often swim freely by flagella.
You can hardly have failed to notice how closely the gastrula corresponds to a hydra, and many facts lead us to believe that the still earlier ancestor of the hydra was free swimming, and that the tentacles are a later development correlated with its adult sessile life. Yet we must not forget that the hydra is even now not quite sessile, it moves somewhat. And our ancestor was almost certainly a free swimming gastraea, or hypothetical form corresponding in form and structure to the gastrula. The ancestor of man never settled down lazily into a sessile life.
But how is an adult worm or vertebrate formed out of such a gastrula? To answer this would require a course of lectures on embryology. But certain changes interest us. Between the ectoderm and entoderm of the gastrula, in the space occupied by the supporting membrane of hydra, a new layer of cells, the mesoderm, appears. This has been produced by the rapid growth and reproduction of certain cells of the entoderm which have migrated, so to speak, into this new position. In higher forms it becomes of continually greater importance, until finally nearly all the organs of the body develop from it. In our bodies only the lining of the mid-intestine and of its glands has arisen from the entoderm. And only the epidermis, or outer layer of our skin, and the nervous system and parts of our
sense-organs have arisen from the ectoderm. But our mid-intestine is still the greatly elongated archenteron of the gastrula.
We may therefore compare the hydra or gastrula to a little portion of the lining of the human mid-intestine covered with a little flake of epidermis. This much the hydra has attained. But our bones and muscles and blood-vessels all come from the mesoderm by folding, plaiting, and channelling, and division of labor resulting in differentiation of structure. Of all true mesodermal structures the hydra has actually none, but in the ectodermal and entodermal cells he has the potentiality of them all. We must now try to discover how these potentialities became actualities in higher forms.
The third stage in our ancestral series is the turbellarian. This is a little, flat, oval worm, varying greatly in size in different species, and found both in fresh and salt water. Some would deny that this worm belonged in our series at all. But, while doubtless considerably modified, it has still retained many characteristics almost certainly possessed by our primitive bilateral ancestor. The different parts of hydra were arranged like those of most flowers, around one main. vertical axis; it was thus radiate in structure, having neither front nor rear, right nor left side. But our little turbellaria, while still without a head, has one end which goes first and can be called the front end. The upper or dorsal surface is usually more colored with pigment cells than the lower or ventral surface, on which is the mouth. It has also a right and left side. It is thus bilateral.
The gastrea swam by cilia, little eyelash - like
processes which urge the animal forward like a myriad of microscopic oars. In our bodies they are sometimes used to keep up a current, e.g., to remove foreign particles from the lungs. The turbellaria is still covered with cilia, probably an inheritance from the gastræa; for, while in smaller forms they may still be the principal means of locomotion, in larger ones the muscles are beginning to assume this function and the animal moves by writhing. The bilateral symmetry has arisen in connection with this mode of locomotion and is thus a mark of important progress.
In the turbellaria we find for the first time a true body-wall distinct from underlying organs. The outer layer of this is a ciliated epithelium or layer of cells. Under this an elastic membrane may occur. Then come true body muscles, running transversely, longitudinally and dorso-ventrally. Between the external transverse and the internal longitudinal layers we often find two muscular layers whose fibres run diagonally. The body is well provided with muscles, but their arrangement is still far from economical or effective.
Within the body-wall is the parenchym. This is a spongy mass of connectile tissue in which the other organs are embedded. The mouth lies in the middle, or near the front of the ventral surface. The intestine varies in form, but is provided with its own layers of longitudinal and transverse muscles, and usually has paired pouches extending out from it into the body parenchym. These seem to distribute the dissolved nutriment; hence the whole cavity is still often called a gastro-vascular cavity as serving both digestion and circulation. There is no anal opening, but indigestible material is still cast out through the mouth.