CHROMOSOME. The entire body of an animal or plant is built up of cells; each cell consists of protoplasm and a nucleus and is usually surrounded by a cell wall; the nucleus consists of chromatin and its derivatives. Dur ing nuclear division the chromatin appears in the form of definite pieces called chromosomes.
This name, meaning color bodies, was given because the pieces stain intensely with certain dyes. The general appearance of chromatin in the resting nucleus is shown in Fig. 1A; Fig. 1B shows the chromatin after it has as sumed the form of definite chromosomes during the division of the nucleus.
Since 1880 investigations dealing with chro matin in the resting condition, but more par ticularly with nuclear division when the chromatin is obviously in the form of chromo somes, have increased in number and in the extreme minuteness of .their details. For a diagrammatic outline of nuclear division and the behavior of the chromosomes during this process, see the article on CELL. In regard to the details of nuclear division, there have been two strongly defended theories, one of which may be called the linin-chrornomere theory and the other the vacuolization theory. The first assumes that the chromatin is in the form of granules rather regularly arranged upon or embedded in a substratum called linin. The granules, or thromomeres, divide and their division splits the linin thread, so that each chromosome produces Iwo new chromosomes (Fig. 2).
All the chromosomes of the nucleus divide simultaneously and are distributed so that each of the two stew nuclei not only receives half of the chromatin of the preceding nucleus, but half of each chromosome. This theory is satisfying to the philosophical mind, but prac tically all botanists have come to the conclusion that it is not supported by the facts. Most botanists accept the vacuolization theory. Ac cording to this theory vacuoles appear in each chromosome and increase both in number and extent until the group of vacuolated chromo somes constitutes the familiar network of the resting nucleus. Thus, there is only one sub stance, the chromatin. The substance which in the other theory is called linin is only the more attenuated portion of the chromatin. As division begins the attenuated portions are withdrawn and the chromatin condenses until the solid chromatin thread appears. This thread splits longitudinally, by some mechanism not yet known; then the thread, already split, breaks up into pairs of chromosomes and one member of each pair goes to each of the new nuclei.
There are many reasons for believing that the chromosome maintains its identity from one cell generation to another throughout the whole life of the plant or animal. The number of chromosomes is practically constant for a given species; e.g., in vegetative tissues, the lily has
24, trillium 12, canna 6, the mole cricket 12 and man has 24. If the nucleus of an egg be re moved and a single sperm introduced the nuclei of the resulting embryo have only half the normal number of chromosomes. If a normal egg be fertilized by two sperms, the resulting embryo shows one-third more chromosomes than the normal. If the two species with dif ferent chromosome numbers be crossed, the resulting embryo shows a number equal to the sum of the numbers found in the sperm and the egg. Some very competent investigators claim they can see the chromosomes outlined in the network of the resting nucleus. Still further evidence might be cited to prove that the chromosome is a permanent organ of the cell.
The number of chromosomes is doubled by the union of the sperm and the egg during fer tilization. At some subsequent point in the life history, before sperms and eggs are formed again, there is a reduction in the number of chromosomes. The most generally accepted view is that the reduction is due to the fusion of chromosomes in pairs. The number becomes reduced during two peculiar, successive divi sions, called the reduction divisions (Fig. 3).
Most botanists and many zoologists believe that the chromosome is the sole bearer of hereditary characters. The male and female at fertilization contribute not only equal amounts of chromatin but, in most cases, the same number of chromosomes; and this is the only equal contribution. In some cases, as in the lily, the male contributes nothing but a nucleus, which consists, essentially, of chromosomes. If the nucleus of an egg be removed and the sperm of another species be introduced the resulting embryo has only the characters of the sperm parent. Other evidence might be added. It has been established that the chromo somes are not all alike and that different chro mosomes may have different functions. In insects, and in some other forms, it has been shown that one chromosome, differing in ap pearance from the rest, is concerned in the determination of sex (See PLANTS, SEX IN). In mutation (q.v.) it has been proved that the chromosome content of the nuclei of the mu tating form differs from that of the parent plant and it seems probable that the change in the chromatin was the cause of the mutation.
Wilson, E. B., 'The Cell in Development and Inheritance' ; Hertwig, O. 'The Cell.' Also many articles since 1895 in various journals, especially The Botanical Ga zette, Annals of Botany, La Cellule, Jahrbiicher fur wissenschaftliche Botanik, Journal of Mor phology and Archiviiir Zellforschung.
CHAnzs J. CHAMBERLAIN, Professor of Morphology and Cytology, Uni versity of Chicago.