It is no part of my present purpose to insist, even to the extent that in this treatise Hertwig himself insists, upon the points of agreement between the two views. We are only at the beginning of inquiry into the problems of heredity, and the protagonists of the opposing views, like all those who care more for knowledge than for argument, are concerned more for truth than for the establishment of a _modus vivendi_. Reconciliation is the parent of slothful thinking and of glosses; it is by sharp contrasting of the opposing views that we are like to have new facts elicited, and new lines of inquiry suggested. As many are interested in the problems who have little acquaintance with the technical facts of embryology, a simple account of the early stages in the development of an animal may be useful for reference. I shall choose back-boned animals, as, from the inclusion of man among them, they are of more general interest. The process begins with the fertilisation of the egg-cell by the fusion with its nucleus of the nucleus or head of a male-cell or spermatozoon. At their first origin the nuclei of the sperm and of the egg may be of very different appearance, while that of the sperm is invariably smaller than that of the egg. But before or during the process of fertilisation, changes take place, the result of which is that the fusing nuclei are exactly alike in morphological character. The chromatin, or peculiar substance of the nuclei, is transformed into a number of bodies known as chromosomes, which are of the same number, form, and size, in the two sexes. Form, size, and number are different in different animals, but there is reason to believe that they are normally the same in all the individuals of a species. The fertilised nucleus, thus consisting of chromosomes from male and female, then divides by a complicated process known as karyokinesis, in which each chromosome splits longitudinally, one half passing to each daughter-nucleus. Throughout the whole process of embryonic and post-embryonic growth, the chromatin is gradually increasing in bulk, and being distributed by karyokinesis. The normal character of these divisions is as follows: A daughter-nucleus, after separation, passes through a resting phase, in which the chromosomes, as definite structures, disappear, and in which growth of the nuclear matter occurs. Then chromosomes of definite size and form, and corresponding in number to those present in the fertilised egg-cell, again appear. These split longitudinally, and a half of each passes to each daughter-nucleus. The similarity of these processes among all living creatures, vegetable and animal, and their extreme complication, suggests that karyokinesis is the chief factor in distributing the hereditary mass to the growing organism. Weismann and some others think that there is evidence for a difference in the nature of the process, which may in some cases correspond to his distinction between doubling and differentiating