Like all living creatures with which we are acquainted, the starting-point of Myriapod life is the _ovum_, as it is called. This _ovum_ is a _cell_ resembling the cells of which the body of all living animals are built up, and which may be compared to the bricks of which a building is composed. This cell or ovum is a small sphere of living transparent substance called protoplasm, and it is _nucleated_—that is, it contains a small spot of denser protoplasm called the nucleus, and within that a still smaller spot of still more dense protoplasm called the nucleolus. In the process of impregnation the ovum unites with the male cell, and the cell so formed is called the impregnated ovum. This ovum has the property of dividing into two cells, each resembling the {64}parent cell from which it is derived; each of these cells has, like the parent cell, the same property of dividing into two more, and so on. Thus from this continual process of division or reproduction of every living cell, the materials are provided for the building up of the body. The regularity of the process of the division of the ovum, or, as it is called, _segmentation of the ovum_, is interfered with by the presence of _food yolk_. The cells formed by the process of cell division just described need nourishment, and this nourishment is supplied to them by the food yolk formed in the body of the ovum before the process of segmentation begins. It is easy to understand that this yolk, which is not _alive_ like the cells, cannot divide like them, and therefore the segmentation of the ovum in Myriapods is _irregular_, as it is called. [Illustration: FIG. 37.—Later stage: _nu_, nucleolus; _c.p_, nucleus; _y.sp_, yolk spherules; _ch_, shell.] I will now go back a little and describe what happens to the ovum before the process of segmentation is complete. It increases in size and forms the supply of food yolk which is to provide the nutriment of the ovum. Then after impregnation the egg-shell is formed round it, and it becomes what we know as the egg. This egg is not a perfect sphere, but is oval (in most Myriapods) in shape. The egg is laid, and the process of segmentation begins shortly after it is laid, as has already been described. When it has been laid for about 36 hours, if we take an egg and, after proper preparation, cut it into thin slices known to {65}microscopists by the name of sections, and examine it by means of the microscope, we shall see that segmentation has resulted in this. Just beneath the egg-shell there is a thin layer of cells, one cell thick, which completely surrounds the egg. Inside this coat of cells is the food yolk, with a few cells scattered about in it at rare intervals, something like the raisins in a plum-pudding.