MULLER'S THEORY AND ITS CONSEQUENCES One principle, which, though no longer regarded as a satisfac factory causal explanation of a great class of facts, has led to invaluable nvaluable zoological work, in the theory formulated by Fritz Muller, that every animal, in its growth from the egg to the adult condition, passes in turn through stages which recapitu late its evolution from a primitive form, or in other words, that ontogeny (the individual development) repeats phylogeny (the development of race).
The attempt to apply this principle led zoologists to investigate in great detail the embryology and later development of some one or more members of each of the greater groups, and to make es pecial efforts to obtain evidence about the life history of those ani mals, which were believed to be of exceptional interest because they stood near to the points of branching of the phylogenetic tree.
These studies, though they can scarcely be said to have been successful in their primary intention, formed the foundation of new developments in morphology, and provided a great mass of facts, which were in themselves of the highest interest, and in many cases have given information of great evolutionary impor tance. (See EMBRYOLOGY.) But embryology was already a sepa rate branch of study before the formulation of the biogenetic law; it arose or was revived during the Renaissance, became of absorb ing interest with the introduction and steady improvement of the microscope and of appropriate technical methods, and has entered on a new lease of life from the application to it of experi mental methods.
Cytology (q.v.), the science which is concerned with cells, be gan with the microscopists of the late r7th and 18th centuries.
Their work, and that of those who succeeded them, led zoologists to the conception that the bodies of all animals (and plants) are either composed of a single mass of living matter, or are com posed of a great number of such units, each independent from, though influenced by its neighbours.
The fact that nearly every Metazoan is, when it first becomes an independent individual, a single cell, renders it certain that an investigation of the structure and other properties of cells must form the basis of any fundamental study of development.
The early recognition by the botanist, Robert Brown, that each cell contained a body, the nucleus, which had a definite shape, and was clearly marked off from the surrounding liquid substance of the cell, and the even earlier discovery of the existence of a streaming of cell substance in plants, and of the formation and migration of food vacuoles in Protozoa, led to attempts to in vestigate the morphology of cells. Such researches became much
easier when methods of fixing (i.e., killing by coagulating the pro teins they contained) and staining cells were invented, and the vast majority of the many thousands of papers on cytology which exist are founded on materials which have been subjected to such treatment.
The greatest single discovery made by cytologists was that of mitosis or karyokinesis, an elaborate mode of multiplication of cells by division into two, which is found in very nearly the same form in all Metazoa and higher plants and in many Protista. Details of this process will be found in the article CYTOLOGY.
Its impoftance lies in the fact that it ensures a qualitatively and quantitatively exact division of the nucleus between the two daughter cells. Mitosis commences with the formation within the nucleus of a long, tangled thread, along which are placed masses of a substance, chromatin, recognized by the intense stain it takes from such basic dyes as haematoxylin. This thread contracts and breaks up into short lengths, chromosomes, which are present in pairs, with the exception that one of them may either differ from its partner in appearance or lack one alto gether.
Subsequently, the membrane which separates the nucleus from the surrounding cytoplasm disappears and the chromosomes are set free. Meanwhile, two small granules, the centrosomes, have placed themselves at opposite poles of the cell, and by their activ ities have altered the nature of the cytoplasm which surrounds them, so that it acquires a radially arranged structure. These rays, proceeding from the neighbourhood of the two centro somes, meet and form a spindle of threads to which the chromo somes become attached. Each chromosome then splits longitudi nally, the halves which result being drawn to the opposite poles of the spindle. From these two aggregates of half chromosomes, two new nuclei are formed and the cytoplasm of the cell is divided into two parts, one associated with each nucleus.