The Maturation Divisions in Relation to the Segregation of Homologous Chromosomes

Abstract

HE discovery of new facts T and their correlations with each other and with those previously known may satisfy the individual research worker, but the practical applications of the facts hold the chief interest for those in other fields of endeavor. Thus, cytology principally interests the public on account of its relation to pathology, to physiology, and to evolution and heredity. Its relationship to the last of these is the especial concern of the present paper. Living things, plant or animal, are either single cells as in the lower forms of life or cobrdinated groups of many cells as in all higher organisms. In other words, the unit of living matter is the cell. This is a microscopical bit of protoplasm which contains a highly specialized, centrally-located part called the nucleus. The essential constituent of the n4cleus is a substance which stains deeply with basic dyes, hence Flemming ('79) named it chromatin from the Greek word xpct/Aa meaning color. Concordant evidence from many lines of work has been sufficient to convince most biologists that chromatin, which is the only material contributed in equal amount by both parents in fertilization, is the physical basis of heredity. The chromatin is in a loose, spongy condition during the ordinary processes of metabolism, but condenses when a cell prepares to divide. It then becomes evident that there is a definite number of dense bodies which were named chromosomes by Waldeyer in i888. During division each chromosome splits lengthwise and a half of every one goes to either new cell. Each species has a definite chromosomal complex, the characteristics of which are most readily distinguished just before the halves of the chromosomes separate during cell division. For instance, the number of chromosomes is practically constant, although certain exceptions exist due to the sex-determining mechanism, supernumeraries and multiple formation. Also a graded size series is usually present. This series is actually double as there are almost invariably two of a given size and shape. Of these two morphologically alike, paired or homologous chromosomes, one has been derived from the male and the other from the female parent. The germ cells in preparation for fertilization undergo a series of changes called collectively gametogenesis. The part of this process with which we are particularly concerned is called maturation and consists of two cell divisions of a somewhat special type. These are known as the first and second maturation divisions. The behavior of the pairs of chromosomes during these two divisions is the especial field of this paper. Before the maturation divisions, homol-