Cytology (CYTOLOGY)

CYTOLOGY (from Greek karyon - nucleus and logos - science), the doctrine of the cell nucleus, has in the present day acquired the status of such a major branch of cytology that it is often (under one title or another) the subject of separate university courses. This is connected, besides the important role played by the nucleus in all life processes, with the exceptional significance attributed to it by modern genetics in the transmission of hereditary Mendelian factors (see Mendelism) and the properties determined by them. Indeed, the correspondence between the requirements imposed by genetics (see) on the cell as the substrate of heredity, and the facts provided by cytology, appears very convincing. Cytology teaches that the nuclei of all cells of all individuals of a given species, which in the resting state may have very diverse size and shape, as well as differ considerably in chromatin content (see), in the overwhelming majority of cases reveal during mitosis the complete identity of their chromosomal set (see Chromosomes), composed of a certain definite number of elements, the relative size, shape, and often observed individual differences of which are constant. The set of chromosomes ("chromosomal complex") represents a kind of cytological characteristic, an idiogram of the species, and as such is often also called a karyotype. All chromosomes (with the exception of the sex chromosomes) in the somatic (body, as opposed to sex) cells of animals and plants are represented by pairs of morphologically identical elements (diploid set); one component of each pair comes from the father, and the other from the mother; the same composition is also characteristic of the complexes of sex cells until their maturation. At all divisions of the fertilized egg cell, which through complex morphogenetic processes lead to the development of the adult organism, this complex remains unchanged; at each division all its components undergo longitudinal splitting. Only at one of the two final divisions accompanying the maturation of spermatozoa and egg cells do peculiarities occur that reduce the number of chromosomes to half (haploid) in mature sex cells. During this time, in the preparatory stage for the given mitosis, which lasts incomparably longer than in ordinary divisions, the paired chromosomes of paternal and maternal origin intertwine with each other, fold together, and stick together pairwise ("conjugation"), and in the then occurring (or following) division ("reductional") they do not split longitudinally, but only the pairs separate, and the former partners go to different poles. This achieves both the haploidity of spermatozoa and egg cells necessary for maintaining the constancy of the chromosome number, and the distribution of chromosomes of paternal and maternal origin in them according to the laws of chance. Besides those already mentioned above, a number of other biological phenomena are found to be closely connected with the functions of the "chromosomal apparatus". Cytological works of the last decades have established this connection for the determination of sex (see), alternation of generations, parthenogenesis, intersexuality, gynandromorphism (see). The results of the latest works in this field prove the existence of parallelism where its establishment recently seemed a task still beyond the powers of cytology. Particularly interesting are cases where the geneticist and cytologist, working by such different methods, mutually confirm the presence of certain anomalies, sometimes extremely difficult to detect and subtle. As an example, one can point to the recent (1929) joint research of the geneticist Muller and the cytologist Painter. Subjecting fruit flies Drosophila (see) to large doses of X-rays, Muller obtained a number of new mutations and besides established by genetic analysis that in these flies under the influence of the mentioned factor, a number of the following anomalies can also appear in the chromosomal complex: sections of varying length can move from one chromosome to another, and such an improper exchange occurs not between paired chromosomes; the middles of chromosomes can be lost, and finally an extra fifth linkage group can appear (the species under study has four pairs of chromosomes). Painter's cytological analysis discovered in the chromosomes of the corresponding animals all the changes expected from the genetic analysis: sticking together of parts of different chromosomes, gluing together of the ends of the same one, and the appearance of an extra chromosome, arising from a fragment that retained its independence. Special attention is deserved here by the fact that in a number of cases it was possible to establish even extremely important quantitative relationships between the size of the moved pieces as observed under the microscope and the size of changes in the position of genes established by their "map" or "plan" (see Genetics) for the corresponding chromosome.