Segmentation

SEGMENTATION (syn. cleavage), an embryological term denoting a series of successive divisions of a fertilized egg cell, as a result of which the egg forms an embryo in the form of a cluster of cells (morula). The cells arising during segmentation are called segmentation spheres, or blastomeres (see). The character of segmentation in the eggs of different classes varies depending on the quantity and distribution of yolk. - Segmentation is divided into: 1. Complete, when the egg disintegrates entirely into blastomeres (holoblastic eggs, with a small amount of yolk). It can be equal (equatorial)—all blastomeres of equal size (mammals), almost equal (adequal)—lower blastomeres slightly larger than the upper ones (amphioxus), and unequal (inequal)—when the lower blastomeres contain more yolk and are significantly larger than the upper ones (macromeres and micromeres) (amphibians). 2. Partial—in eggs with a large amount of yolk, when only the portion of the egg rich in protoplasm is divided, forming the blastoderm (see), while the main mass containing yolk remains undivided (meroblastic eggs). Types of segmentation: 1) discoidal segmentation—the blastoderm forms a plate or disk on one side of the egg (fishes, reptiles, birds)—and 2) superficial (superficial)—the blastoderm surrounds from the surface the central undivided mass of yolk (insects). In discoidal segmentation the undivided mass of yolk subsequently begins to form cells, joining the blastoderm; this process occurs with the participation of nuclei arising from the blastoderm (yolk nuclei, merocytes) and is called secondary segmentation. In all types of segmentation the first segmentation furrow usually runs along the meridian of the egg, the second also along the meridian, perpendicular to the first, the third perpendicular to both, along the equator. In the future such a regular course of segmentation may be preserved; as a result, blastomeres in each layer from top to bottom are arranged directly one above the other, and the morula exhibits radial or bilateral symmetry (radial and bilateral segmentation). However, in a number of invertebrates (mollusks) spiral segmentation is observed: blastoderms of the lower layer are placed in the intervals between the blastoderms of the upper layer and are arranged (if going from top to bottom) along a spiral line. This is conditioned by the oblique direction of the plane of segmentation. The direction of furrows and planes of segmentation is determined by Hertwig's law: 1) the plane of segmentation is always perpendicular to the axis of the spindle of the dividing blastomere, 2) the poles of the spindle always establish themselves in the direction of the greatest accumulation of protoplasm.

v. carpov. YEASTS (Saccharomycetes), microscopic fungi that do not form a typical mycelium and have the appearance of cells or single or connected into small chains. Reproduction is primarily by budding, and in some cases by cell division. Unlike yeast-like fungi similar in appearance, all true yeasts possess the property of forming endospores in their cells under certain conditions, which allows them to be brought close to simple ascomycetes (order Protascineae; see also Blastomycetes). Several hundred species of yeasts are known; some of them cause alcoholic fermentation (see) in saccharine liquids and are known in everyday life under the names of baker's, beer, wine, and other yeasts; in most, however, this ability is absent or expressed to a very slight degree. Besides true yeasts, this name is sometimes given to various other fungi that can cause alcoholic fermentation and are used for this purpose by man. For example, "East Asian yeasts" (Mucor Rouxii, Mucor javanicus) and others, which form a true mycelium. Botanically they belong to a completely different group of fungi (to the zygomycetes). - In Europe and America, as causative agents of alcoholic fermentation, almost exclusively certain species of the genus Saccharomyces are used, mainly Sacch. cerevisiae and Sacch. ellipsoideus. Of these, Sacch. cerevisiae—beer yeasts—are especially important. They occur only in a cultured state and are known in the form of several races, distinguished both by small morphological signs and by certain physiological peculiarities. They are used not only in brewing but also in bread making (compressed yeasts, mainly races XII and XV) and in distilling. Sacch. ellipsoideus (wine yeasts) are bred in the form of pure cultures, but they also occur in a wild state in the soil of vineyards, from where they get onto the surface of berries, where they are always present in significant quantities. They are known in the form of several races, determining to some extent the bouquet of the wine obtained. In view of the fact that wild yeasts usually consist of various races possessing the most diverse properties, and in addition contain admixtures of various bacteria and molds, the fermentation caused by them does not always proceed favorably, and the fermentation product may have a poor taste or smell. In order to avoid this and to be always sure of a good outcome of fermentation, in winemaking, brewing, and in bread making it is necessary to use only pure cultures of yeasts of certain races. - Yeast cells contain 70% water, 13% proteins, 14% easily soluble carbohydrates, mainly glycogen. Fat in young cells is less than 1%, in old cells significantly more. In some special, so-called "fatty yeasts" (Endomyces vernalis) the fat content reaches 30%. Therefore yeasts can serve as a concentrated feed for animals.

L. Kursanov. In medical terms, segmentation is of interest both for diagnosis and for therapy. In gastric contents, upon microscopic examination, segmentation is detected in the form of round or elliptical dots, sharply refracting light, located singly, in pairs, or in chains (budding). The latter form, along with sarcinae, is observed in large quantities during prolonged retention (stagnation) of acidic contents in the stomach, for example in connection with benign pyloric stenosis. Lactic acid fermentation in stomach cancer with achylia seems to hinder the development of sarcinae and to some extent segmentation (Boas). In this sense, the presence of segmentation in the sediment can have some differential-diagnostic significance. For therapeutic purposes, beer yeasts (both in fresh form and in the form of a dried preparation) are applied in furunculosis, pyoderma, and other purulent processes and diseases where there are grounds to assume disorders of metabolism or auto-intoxication. The mechanism of action of yeasts remains unclear. Apparently due to the presence of fermenting enzymes in them, yeasts hinder the development of pathogenic microorganisms in the intestine. In addition, yeasts themselves possess bactericidal and antitoxic action. Finally, yeasts are used in the preparation of kumys (see Kumys, kumys treatment).