Page:Popular Science Monthly Volume 71.djvu/463

Rh the size of the nucleus, some further explanation is necessary, and that can best be given with the aid of some illustrations upon the screen. The first of the pictures will, I hope, serve to recall to your minds what I said in regard to the process of the segmentation of the ovum. Here is an ovum, No. 1, a single cell, but relatively of enormous size, the ovum or germ of a newt, and the plate illustrates to us the gradual process of division of the original single cell into a number of distinct cells, and each of these we call a segment, and the formation of them, segmentation, a name which we keep from the olden time when the process was first observed by some French investigators, because it is so descriptive of the appearance presented to the eye by the changes which are going on. Were we to name the process now we should certainly call it a process of cell production.

The next of our pictures shows us the eggs of a common snail, the Planorbis, a little fresh-water snail, the coils of which lie flat in one plane—hence its name. No. 1 is the original germ; No. 2 shows it about to divide into two; No. 3 is a side view; No. 4 a top view of the ovum with two segments; No. 5 is cleft into four segments; No. 6 into eight. Nos. 7 and 8 illustrate the further progress of the cell multiplication; No. 9 represents the under side of the same egg of

which the top is figured as No. 8. The number of cells (segments) is thus constantly increasing and already it is evident that they have become somewhat unlike in character. Were the picture still further magnified, we could see that in these cells a change is going on in the nuclei which, however, I can better demonstrate to you by means of the following picture, one which we saw in the last lecture. These are sections through the early developing germ of a mammal named Tarsius spectabile. It is a creature nearly related to the lemurs, having