Page:Popular Science Monthly Volume 71.djvu/470

464 small insect, a mosquito or a little beetle, whatever it may be, is not big enough to have a great many cells; and, unless it has a great many, it can not attain the differentiation of complicated organs such as we possess. Now, the lower animals are born, so to speak, early, and as soon as they hatch out. they have to support themselves. We see that, for instance, in caterpillars. They are born very little creatures, but each caterpillar must look out for itself, obtain its own food, move about to that food, must, when the food is swallowed, digest it, and must carry on the correlated functions of secretion and excretion; it must breathe. In order to do all this the larva, or young caterpillar, to follow our special instance, must have some differentiation already established; but, as we have already learned, differentiation impedes growth. In other words, in such a larva the multiplication of cells is held back by the very demands of the conditions of its existence. If it is to have organs which are to function, it must have differentiated parts, and, if it is differentiated, its growth power must be sacrificed.

Now how has nature proceeded in order to produce a higher type of animal, one in which the number of cells is much greater? Very ingeniously. She provides the developing organism with a food supply which it carries itself. If, for instance, you recall the egg of the salamander, which I showed you upon the screen, you will remember that that is a structure of considerable size, and its size is due to the accumulation of food material, material which we designate by the term yolk granules, which lie in the living protoplasm of that germ. This supply of food is so great that it will last the organism a considerable period. While it is growing it has nothing to do but to digest that food supply which it already possesses. It does not have to exert itself to obtain it, and no further digestive process is necessary than that inherent in all living protoplasm. So the young salamanders develop in a most advantageous condition, and can actually produce a much greater number of cells because it is possible, with this internal food supply, for the growth to go on only with the cells of the embryonic or youthful type for a considerable period, and then, when their number has considerably increased, steps in the process of differentiation.

In the higher animals this accumulation of food for the nourishment of the germ is carried yet further. As you know, the egg of the bird is much bigger than that of the salamander, and in the highest animals, in the mammals, there are other special contrivances which nature has introduced to secure ample and adequate nourishment of the developing germ. There the perfection of the process is made yet greater and in these forms there is a long period during which the production of cells goes on; the cells all remain simple, and by the time they begin to change the number of cells is so great that the possibilities of an infinite variety, almost, of peculiarities in them are given, and