In these highest forms of life, however, the cell itself is becoming a much more complicated thing than that lowly form which we first noted as dividing into two to form two new individuals. Indeed, the cells in the highest animals and plants are immensely complicated in their structures and functions, and especially in connection with the changes which take place in the nucleus of such cells. Not only the nucleus but another small object within the cell which is neither part of the nucleus nor part of the cell protoplasm, also is very important, and this structure is termed the "centrosome." In fact this little body apparently begins the whole process of cell-division by itself dividing into two parts. Then the nucleus follows suit, and ultimately the whole cell divides. The nucleus itself is a complicated structure, as is especially seen during the processes of division, in which it breaks itself up into a number of thread-like portions, and the number of these is always the same in any given species, a fact which is of great importance in reproduction. Why do we mention these apparently dry details? Because in these minute and complicated nuclear movements the whole problems which are at the bottom of development and heredity lie. The problems of life itself can only be solved by the study of what takes place in these minute portions of cells. It is here that the new formation of an individual begins, and although it is no part of our purpose here to detail all the complicated processes of nuclear division, it is essential, in order to grasp the meaning of our subject, that we should realize that in the changes within the cell life with its variations begins. The study of these wonderful cell processes, a work which demands the most patient investigation and high technical skill, has reached such a stage that it is a science of its own, and is called the science of "Cytology," or the science of cells, which has been made possible only in comparatively recent years by the invention of microscopes having great powers of magnification, and by the application of elaborate methods of staining to the cells themselves. We can say no more about these processes here, but the foregoing paragraphs may perhaps be sufficient to show us how important it is to grasp these simple facts of cell life in their bearing upon development itself. CHAPTER II PROBLEMS OF REPRODUCTION