Publisher Summary

It took some time until the mechanism that brought about the halving of the chromosome number during meiosis became understood; by the end of the 19th century, the basic phenomena of the cell were known. The bodies of all higher animals and plants are composed of cells, which have originated by the repeated division of the fertilized egg. The characters of the parents are transmitted to the offspring by means of the chromosomes that are situated in the nuclei of the egg and the spermatozoa. Prior to the formation of the gametes, the number of chromosomes that are present in the body cells of the parents is reduced by one-half, and the original number is restored at fertilization. The process of mitosis ensures that all newly formed cells receive the same complement of chromosomes. Mendel’s results were not merely in agreement with breeding experiments in progress at the turn of the century; the similarity shown in the behavior of Mendel’s factors and that of the chromosomes was decisive in establishing the theory that the chromosomes are the bearers of the hereditary material.

I Introduction

It was in 1908 that Edmund B. Wilson gave an address to the American Association for the Advancement of Science. The subject was: “Recent researches on the determination and heredity of sex,” and in this talk he asked the following two questions: “Does sex arise, as was long believed, as a response of the organism to external stimuli? Or is it automatically ordered by internal factors, and if so, what is their nature?” He concluded that in all probability sex was controlled by internal factors of the germ cells, and that the male or female condition does not arise primarily as a response of the developing organism to corresponding external conditions (Wilson, 1909a).

This answer was by no means self-evident. Twelve years before, Wilson himself had held that “the determination of sex is not by inheritance, but by the combined effect of external conditions” (Wilson, 1896). At the close of the nineteenth century the view prevailed that the embryo was at first sexually undifferentiated and that sex was subsequently determined by such agents as temperature and nutrition (Wilson, 1896; Doncaster, 1914). Let us therefore retrace some of the landmarks which caused this fundamental change in outlook.

II The Development of the Cell Theory

During the course of the nineteenth century the role of the cell as the basic unit of organisms became gradually understood. The detailed study of cells required the existence of optically advanced microscopes, and such instruments became available in the second quarter of the century. The introduction of achromatic lenses at that time paved the way for new investigations and discoveries (Nordenskiöld, 1927), while conversely a renewed interest in microscopic observations encouraged improvements in the instruments and their production in larger numbers (Hughes, 1959).

In 1833 the Scottish botanist Robert Brown published his discovery that cells contain a nucleus as an essential component. A few years later, the cell theory was put on its feet by Schleiden (1838) and by Schwann (1839), in Germany. They established that animals and plants are organized into basic units of comparable structure, which have an individual life and yet coordinate to form the organism as a whole. The tissues of the body are composed either entirely of cells, or of cells plus products which originated in cells.

As regards the origin of new cells, Schleiden and Schwann had thought that this might come about by either one of two processes. Cells might arise either from a parent cell, or they might be formed by a process of free cell formation, crystallizing from a material which was not itself composed of cells. Gradually it became clear that the idea of free cell formation had to be abandoned. In his book on cellular pathology, published in 1858, Rudolph Virchow insisted that every cell must be the offspring of a preexisting cell, just as an animal arises only from an animal or a plant from a plant; and gradually the concept of the continuity of cells from generation to generation was established.

III Mitosis

Although the principle that cells arise only from preexisting cells has become the foundation of modern biology, when Virchow wrote this, evidence was, as yet, unavailable. Indeed, the mechanism by which cells divide eluded investigators for another 20 years. Then, during the 1870s began an era of intense investigations into the problems of cell division and fertilization. The introduction of techniques for fixation and staining made it possible to study the different processes in considerable detail; and new discoveries followed each other in close succession. In 1873, Schneider announced that during cell division the nucleus does not disappear, as had hitherto been assumed, but undergoes a complicated process of metamorphosis. By the end of the decade, four investigators reported that they had succeeded in following the process of cell division in living cells. Flemming (1879) saw it in epithelial cells of salamander larvae, Peremeschko (1879) in epithelial cells of newt larvae (Triton cristatus), and Schleicher (1879) in cartilage cells of amphibian larvae, while Strasburger (1880) described it in the staminal hairs of the spiderwort (Tradescantia virginica). Thus, the sequence of events could be verified, and it became clear that essentially the same process of cell division occurs in animals and in plants (Flemming, 1882a; Strasburger, 1884).

The last decade of the nineteenth century saw the introduction of apochromatic lenses, which removed the residual chromatic aberration inherent in the achromatic combinations (Hughes, 1959); and thus, the resolving power of the microscope had reached the highest degree possible with visible light.

Schleicher (1879) called the process of cell division “karyokinesis” (nuclear movement), a term which is still sometimes used; it is of interest because it recognizes the kinetic nature of the nucleus, in which stages of quite different appearance give rise to one another. Flemming (1879, 1880) made the all-important discovery that the threads, into which the nucleus resolves itself prior to cell division, divide lengthwise, and van Beneden (1883) and Heuser (1884) showed that in both animals and plants one member of the two newly formed threads went to each daughter cell. Flemming also introduced the word “mitosis” (1882a), as well as the term “chromatin” to denote the substance in the cell nucleus which takes up the color from nuclear dyes (1880). The word “chromosomes” is due to Waldeyer (1888) : “I should like to permit myself the suggestion that those bodies, which Boveri has called ‘chromatic elements’ and in which occurs one of the most important acts of karyokinesis, i.e. Flemming’s lengthwise division, be given a special technical term, ‘chromosomes,’ ”*

IV Fertilization

Once the fundamental aspects of the cell were appreciated, it was at last possible to understand the facts of fertilization, and its significance. Oscar Hertwig (1876) observed, among others, the eggs of the sea urchin, Toxopneustes lividus, which are particularly favorable objects for study, since they are transparent and can be artificially inseminated. He discovered that during fertilization two nuclei unite, one of which is derived from the egg and the other from the spermatozoon, and he concluded that fertilization consists in the fusion of sexually differentiated cell nuclei. Having established this, Hertwig went one important step further: Since fertilization must be the act during which the qualities of the father are transmitted to the offspring, he concluded that the nuclear material must be the bearer of those qualities which are inherited from parents to children (Hertwig, 1885).

These conclusions were confirmed by the work of van Beneden (1883) on the fertilization of the threadworm, Parascaris equorum, (formerly Ascaris megalocephala), which lives as a parasite on the horse. In Parascaris, the eggs are transparent, and the organism has the further advantage of having a small number of large chromosomes; van Beneden was able to observe that in the variety which he studied the sperm and the egg nucleus each resolve themselves into two chromosomes and that the four chromosomes then divide longitudinally, so that each daughter nucleus receives equal amounts of paternal and maternal chromosomes. Thus, it became clear that the male and the female germ cells are equivalent from the point of view of the hereditary material which they contain and that each germ cell contributes one-half of the chromosomes that are present in the offspring.

Van Beneden’s results on Parascaris were confirmed by Boveri (1890), who also extended them to a number of other animals; he showed that in the sea urchin Echinus microtuberculatus, each parent contributed 9 chromosomes, in the worm Sagitta bipunctata, 9, in the medusa...