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The human embryo begins its very existence as a single cell, nothing more and nothing less; in general structure the human egg, like the eggs of all other many-celled organisms, is just one of the unitary building blocks of the entire organic world. And yet the egg may ultimately become the adult man.
The fertilized egg-cell, formed by the union of egg and spermatozoon, is a single cell, like the individual resulting from the conjugation or fusion of two protozoa. But in the many-celled individual, which develops out of the fertilized egg, there are two kinds of cells. 1.
There is danger of what in our bodies would be called suffocation and blood-poisoning. The amoeba having attained a certain size meets this emergency by dividing into two small individuals, the division is a physical adaptation. But the many-celled animal cannot do this; it must keep its cells together. It gains the additional surface by folding and plaiting.
In this we have a clue to the next stage of animal development. Take a soft india-rubber ball to represent the first many-celled animal. Press in one half of the ball close upon the other, narrow the mouth, and you have something like the body-structure of the coral and hydra.
One step higher in the scale stands the plant-form called Volvox, near the border-line between the one-celled and the many-celled organisms. This aquatic type, about the size of the head of an ordinary pin, is a hollow spherical colony, with a wall composed of closely set cellular components.
We have an even greater difficulty here, as plant remains are not preserved at all until the Devonian period. We can only conclude, from the later facts, that these primitive many-celled plants branched out in several different directions.
The fruit is a two to many-celled berry with central fleshy placenta and many small kidney-shaped seeds which are densely covered with short, stiff hairs, as seen in Figs. 3 and 4. It is comparatively easy to define the genus with which the tomato should be classed botanically, but it is by no means so easy to classify our cultivated varieties into botanical species.
=Characteristics of the fruit.= The fruit of the original species from which our cultivated tomatoes have developed was doubtless a comparatively small two to many-celled berry, with comparatively dry central placenta and thin walls. In some species the cells were indicated by distinct sutures, forming a rough or corrugated fruit.
It may therefore be useful to consider one or two intermediate forms and the parallel embryonic stages of higher animals, and to see how the higher many-celled animal originates from the unicellular stage. The amoeba is an illustration of a great kingdom of similar, practically unicellular forms, which have played no unimportant part in the geological history of the globe. These are the protozoa.
The human person, like every other many-celled individual, IS BUT A PASSING PHENOMENON OF ORGANIC LIFE. With its death, the series of its vital activities ceases entirely, just as it began." That certainly is discouraging to a man who for fifty years has sung "I want to be an angel." Yet that is what Haeckel has to say about our chance of immortality.
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