divisions, but it may be said at once that the record of observations is yet too conflicting for any such general interpretation. Along with the increase in bulk and distribution of the nuclear matter, there goes an increase in bulk and segregation of the ordinary protoplasm. The simplicity of the actual development of most back-boned animals is disguised by provision for the nutrition of the growing embryo. In a large number of cases, as, for instance, in birds and reptiles, the egg-cell, a microscopic structure at its first formation, is bloated out into the large eggs with which we are familiar, by the addition of quantities of food-yolk. These eggs, although morphologically single cells, do not divide as cells. A small disc of protoplasm, surrounding the nucleus, floats upon the surface of the yellow yolk, and, when the nucleus divides, furrows appear in this between the daughter-nuclei, but stretch very little way into the inert food-yolk. The subsequent marshalling of the cells is disguised by their association with a preponderating mass of inert material. In a far-distant period in the history of evolution, the eggs of mammals like man were large, and contained, as in the lowest existing mammals, a store of food-yolk. Now the food-yolk is not formed, as the developing embryo obtains its nourishment from the blood of the mother. But the course of development is distorted, partly as a legacy from the old large-yolked condition, and still more to suit the new method of nutrition. Some of the simpler animals even among existing vertebrates still exhibit a marshalling of cells common among invertebrates, and to be traced under the complications of higher forms. In these, now, as in the marine ancestors of all the vertebrates, the fertilised egg is a tiny cell provided with very little yolk, and set adrift in the sea-water. The first division of the nucleus, and each subsequent division of the daughter-nuclei, is at once followed by division or _segmentation_ of the whole cell. The plane between the two cells thus formed is called the first cleavage-plane, and is regarded as vertical. The second cleavage-plane is at right angles to the first, and is also vertical, so that the little embryo consists of four cells, all on the same horizontal plane. The third cleavage-plane is horizontal, and divides the four cells into an upper and lower tier of four cells. In the course of a series of divisions the eight cells come to form a hollow sphere--the blastosphere--enclosing a cavity known as the cleavage or segmentation cavity.