embryological development of the individual. The mass of germplasm derived from the germplasm of the parent lies in a mass of ordinary protoplasm. Both the protoplasm and the germplasm, by the assimilation of food, gradually increase in bulk until the adult size of the organism is reached. Along with the increase of size there occurs a gradual specialisation, during which the tissues, organs, and structure of the creature are attained. The simplest conception of this process is to regard the initial mass as a single cell, the nucleus of which is composed of the parental germplasm. The nucleus and the protoplasm increase in size, and then, first the nucleus and next the protoplasm divide, so that there are formed two cells, each with a nucleus. Each of these again divides, and the process goes on continuously, the new-formed cells gradually being marshalled into their places to form the adult tissues and organs, and they gradually assume the special characters of these tissues and organs. Now, Weismann's theory supposes that the first division of the germplasm is what is called in this translation a _doubling_ division (_Erbgleiche Theilung_). The mass has grown in bulk, without altering its character, so that each resulting mass is precisely like the other. One of the two portions subsequently increases in bulk, and may again divide repeatedly, but always by doubling division. It therefore remains unaltered germplasm, and eventually is marshalled to the part of the adult from which new organisms are to arise, becoming, for instance, in the case of a woman, the nuclear matter of the ovary. Thus, the germplasm is handed on continuously from generation to generation, forming an unbroken chain, through each individual, from grandparent to grandchild. This is the immortality of the germ-cells, the part of the theory which has laid so strong a hold on the popular imagination. And with this also is connected the equally celebrated denial of the inheritance of acquired characters. For, at first, it seemed a clear inference that, if the hereditary mass for the daughters were separated off from the hereditary mass that was to form the mother, at the very first, before the body of the mother was formed, the daughters were in all essentials the sisters of their mother, and could take from her nothing of any characters that might be impressed upon her body in subsequent development. As this treatise touches only indirectly on the question of acquired characters, it is necessary only to mention that while his early sharp denial of the possibility of inheritance of acquired characters has led to a damaging criticism of supposed cases, Weismann, in the riper development of his theory, has found a possibility for the partial transference of influences that affect the mother to the germplasm contained within her.