The More Precise Investigation of the Material Basis of Inheritance.--The application of the microscope to critical studies of the structure of the germ-plasm has brought important results which merge with the development of the idea of germinal continuity. Can we by actual observation determine the particular part of the protoplasmic substance that carries the hereditary qualities? The earliest answer to this question was that the protoplasm, being the living substance, was the bearer of heredity. But close analysis of the behavior of the nucleus during development led, about 1875, to the idea that the hereditary qualities are located within the nucleus of the cell. This idea, promulgated by Fol, Koelliker, and Oskar Hertwig, narrowed the attention of students of heredity from the general protoplasmic contents of the cell to the nucleus. Later investigations show that this restriction was, in a measure, right. The nucleus takes an active part during cell-division, and it was very natural to reach the conclusion that it is the particular bearer of hereditary substance. But, in 1883, Van Beneden and Boveri made the discovery that within the nucleus are certain distinct little rod-like bodies which make their appearance during cell-division. These little bodies, inasmuch as they stain very deeply with the dyes used in microscopic research, are called chromosomes. And continued investigation brought out the astounding fact that, although the number of chromosomes vary in different animals (commonly from two to twenty-four), they are of the same number in all the cells of any particular animal or plant. These chromosomes are regarded as the bearers of heredity, and their behavior during fertilization and development has been followed with great care. Brilliant studies of the formation of the egg have shown that the egg nucleus, in the process of becoming mature, surrenders one-half its number of chromosomes; it approaches the surface of the egg and undergoes division, squeezing out one-half of its substance in the form of a polar globule; and this process is once repeated.[8] The formation of polar globules is accompanied by a noteworthy process of reduction in the number of chromosomes, so that when the egg nucleus has reached its mature condition it contains only one-half the number of chromosomes characteristic of the species, and will not ordinarily undergo development without fertilization.