The philosophical question of constancy and change, as applied to the proteins of the animal body, has been a subject of controversy for over half a century. The experiments of Folin (1905 a) on the effect of the protein level of the diet on the composition of urine marked the beginning of modern concepts of protein metabolism. On the basis of his experiment, Folin (1905 b) assumed that protein catabolism is not all of one kind, but could proceed via two pathways which are essentially independent and different. One pathway of catabolism is variable in quantity of end products, while the other process is constant.

The science of histology endeavours to achieve a knowledge of the detailed morphology of normal tissues. It is also concerned with relating the structure and function of biological (morphological) units at the microscopic level to their internal and external environments.

Modern histology is concerned principally with attempts to correlate the precise order which exists in the organization of organs and tissues at a microscopic level with the biological functions of these structures. Out of the increased need for knowledge of such structural functional relationships has grown the branch of histological study known as histochemistry. Its researches are dedicated to the development of new and useful chemical, physical and biological techniques which may be applied at the cellular or tissue level for the elucidation of such varied functions as, for example, enzyme action, secretion, or reproduction.

The physiological role of the lung was not realized before the middle of the sixteenth century. In 1553 Michael SERVETUS stated that there occurred in the long "the generation of the vital spirit, a compound of the inspired air with the most subtle portion of the b1ood. 1) The first investigations on the intimate structure of the lung were those by MALPIGHI (1697) who demonstrated that the two systems in the lung, the air conducting and vascular systems, were independent and that no communication existed between them.

Before the PA-Schiff technique was developed, silver impregnation was an effective method for the histological demonstration of reticular fibers and basement membranes (Mareseh, 1905; Herxheimer, 1907; Kon, 1908 etc.). The PA-Schiff technique, developed by McManus (1946), Lillie (1947) and Hotchkiss (1948) shows reticular fibers and basement membranes very beautifully (Lillie, 1947; KeManus, 1948 (a) and (b); Leblond 1950) and was believed to indicate in tissue sections the presence of polysaccharides tree or combined with protein (G1egg et a1, 1952).

Labeled thymidine which is known to be a precursor of DNA may serve in the classical function of radioactive precursors - to indicate the time and place of synthesis of DNA. But due to the special properties of DNA, the usefulness of labeled thymidine may be vastly increased. DNA is perhaps the most stable substance within a cell, and unless the cell undergoes mitosis, the same molecules of DNA will remain in it indefinitely. If these molecules happen to be labeled with radioactive thymidine, the cell can be traced by this label for weeks or months and its fate thus made apparent.

The first description of the microscopic anatomy of the thyroid gland was given by Lalouette (1750) who described it as being composed of inter-communicating vesicles. However from time to time controversies arose over different aspects of thyroid histology. Jones (1852) and more recently other workers (Bargmann, 1939; Maximow and Bloom, 1957; etc.) described the thyroid gland as being covered with a connective tissue capsule from which septa penetrated the gland and divided it into well-defined lobules. Bichat (1823) and others (Wegelin, 1926, etc.) failed to observe the limits of the lobules distinctly.

The present investigation is an attempt to gain an insight into the problem of the formation of the carbohydrate moiety of tissues of mesenchymal origin using radioautographic and histochemical methods. Various tissues and organs of the growing rat were studied, but dentin was chosen as the organ most suitable for detailed study, for reasons which will be stated in a later part of this presentation.

The perpetuation of the species has always been the concern of mankind, and especially of biologists. It was not suprising therefore, that with the advent of the histological technique in the mid-nineteenth century, the origin and continuity of the germ cell line, and the development and structure of the reproductive organs became of great histological interest. Thus it was that the origin and development of the sex cords, and the origin and development of primordial germ cells received extensive coverage in the literature. The problem of the primordial germ cells has received thorough and exhaustive treatment in many mammalian species.

Note: Missing title page. / The fundamental importance of desoxyribonuoleic acid (DNA) in cytophysiology is readily understood when it ia realized that this substanoe is a major constituent of chromosomes. Its location in these bodies corresponds to the loci of the genes, the carriers ot the hereditary characters (15,76,97,104,105).