Page:Popular Science Monthly Volume 71.djvu/381

Rh natural thought fails to occur to us. We are very dull even if we are scientific.

The pictures now before you represent certain early stages in the progress of development of a mammal by the name of Tarsius, a creature related to the lemurs. The various figures illustrate the multiplication

of the cells. That which I wish to call your attention to can be well demonstrated by the comparison of the first figure, in which there is a single nucleus, with the figure at this point having a number of nuclei. Both figures represent the very earliest stages of development and show the full size of the whole germ, which is about the same in the two stages. The total amount of living material has not changed essentially, but evidently there has occurred a marked increase of the nuclear substance. The nuclei have in the right-hand figure multiplied in number and their combined volume is much greater than the total volume of the single nucleus in the left-hand figure.

We can get a further notion of the nuclear increase by studying the very early development of a salamander. Here upon the screen is the egg of a salamander. It represents really but a single cell. It then divides into two cells; each of those cells has a nucleus which we can not see because these pictures are taken from the living egg, and the living egg is not transparent. Here it is dividing into four, here the upper portion of the four cells has been split off, and we have seven cells showing in the figure, and an eighth on the back. Here the number of cells has increased very much, and as you view these figures you will notice that they look very much indeed like oranges divided into segments. It seems, in fact, as if this egg, which was spherical in form, were being divided up into a certain number of segments. The process was first observed in the eggs of some of the amphibia, frogs, toads and salamanders, and it was therefore called segmentation, because it was not known at that time what the process really meant. We have then before us an ovum and a series of stages of the segmentation of the ovum, and the result of that segmentation is to produce an