Egg

EGG, the germ cell of multicellular organisms. Depending on the stage of development, a distinction is made between immature eggs (oogonia, 1st and 2nd order oocytes), mature eggs (capable of fertilization), and finally, fertile eggs—fertilized, developing eggs of mammals. The shape and size of eggs vary within extremely wide limits; as a rule, however, it can be said that the eggs of many animals are among the largest of cells. Many eggs are clearly visible to the naked eye and sometimes reach very significant sizes (eggs of birds, reptiles, amphibians). The first data on the structure of the egg are found in the old works of the 17th and 18th centuries (Harvey, de Graaf, and others), but the initial interpretation of the pictures observed here was in many ways incorrect; thus, de Graaf considered the so-called Graafian follicle, which is a complex system of membranes enclosing a small egg inside, to be the mammalian egg. Only in the middle of the 18th century did Caspar Wolff describe the chicken egg in detail. The mammalian egg was studied much later (first by K. E. von Baer in 1827). The structure of the egg essentially depends on the amount of nutrient substance enclosed within it—the yolk (deutoplasm, paraplasm). Eggs are distinguished as: 1) alecithal—practically devoid of yolk (mammals, amphioxus); 2) isolecithal—with a very uniform distribution of yolk (echinoderms); 3) telolecithal—with a large amount of yolk gathered at one (lower) pole of the egg (many invertebrates, selachians, amphibians, reptiles, birds). In telolecithal eggs, a distinction is made between the upper animal pole, where the egg cytoplasm is concentrated, and the vegetative pole, filled with heavier lumps of yolk. In some insects, 4) centrolecithal eggs are found—protoplasm in the form of a sheath covers a significant mass of yolk lying in the center of the egg. Yolk represents granules, grains, or plates of various sizes and shapes. Its arrangement in the egg is also very different—in some cases it uniformly fills the entire protoplasm, in others it is located in the center or along the periphery of the egg. The nucleus of the egg undoubtedly takes part in the formation of the yolk; in some cases (in Dytiscus, Proteus, and others) it is possible to show the emergence of chromatin grains from the nucleus and the deposition of the first yolk grains around them (Fig. 1). The rather enigmatic organelle of the egg cell—the so-called yolk nucleus, or accessory nucleus, first described by Balbiani in the cells of a number of invertebrates—also apparently has a relation to the formation of the yolk. It arises at the expense of nuclear substances and is a complex formation (Fig. 2) with a lighter central part and concentric dark areas on the periphery.

Fig. 1. Nucleus of the egg of Pholcus phalangioideus during yolk formation; the longest pseudopodia are directed toward the dense accumulation of yolk. (After van Bambeke from Korschelt and Heider.)

The protoplasm (cytoplasm) of the egg free from inclusions is usually represented by small layers of a spongy or homogeneous structure. The nucleus of the egg cell throughout the longest stages of its development has the appearance of a vesicle (hence the name vesicula germinativa of old authors—Purkinje, 1824), extremely poor in chromatin. The shape of the egg nucleus is usually round or oval; in many cases, it is possible to note the appearance of pseudopodia-like outgrowths directed toward the site of yolk plate deposition in the egg, and sometimes the nucleus moves entirely to the periphery of the egg into the region of nutrient penetration. Nucleoli (one, two, or several) are clearly visible inside the nucleus. One of them is distinguished by a special stainability (macula germinativa) (Wagner, 1834). Its significance in the biology of the egg is unclear. There is a number of organelles in the egg—the Golgi apparatus, chondriosomes, and centrosomes. In the eggs of amphibians and some echinoderms, a pigment is present, localized in certain parts of the egg and marking the areas from which certain organ systems of the embryo will develop. Egg membranes can be divided into 3 groups. The primary membrane, the so-called vitelline membrane, oolemma, or zona pellucida of mammalian eggs, is a thickened outer edge of the protoplasm; in some cases, it has a complex structure; it is provided with pores through which nutrients can pass into the egg. The time of appearance of the vitelline membrane is very different; sometimes (mammalian eggs) it arises before fertilization, and then a special opening for the penetration of spermatozoa—the micropyle—is present in it; in other cases (echinoderms), the vitelline membrane arises only after fertilization and the penetration of the first spermatozoon into the egg.

In some animals and plants, the egg cell is surrounded by a group of poorly differentiated epithelial cells (follicular cells). The significance of these cells is very diverse; most often they transmit nutrients to the egg (nurse cells), and in some cases, the absorption of these cells, as well as of non-viable egg cells located in the same membrane with them, by the developing egg is noted. Follicular cells secrete a dense membrane, the chorion, on the surface, reaching significant thickness in some cases.

A third kind of membranes is secreted outside the gonad during the passage of the egg through the genital tracts, oviducts, uterus, etc. Very interesting examples of formations of this kind can serve as insect cocoons and vertebrate egg membranes (fish, amphibians, reptiles, birds). Here takes place the deposition of additional reserves of nutrients (chicken egg white, protein fluid in the cocoons of many invertebrates), which the long-developing embryo must feed upon. The material of such formations is very diverse—it is chitin in invertebrates, a dense substance in lower vertebrates, a lime-impregnated hard shell in birds and some reptiles. The chicken egg is an extremely complex formation. Besides the germinal disc itself, i.e., the patch of pure protoplasm located at the animal pole where the embryo develops, and a huge amount of yolk, a number of additional parts are distinguished. Such are the concentrically located areas of white and yellow yolk, differing from each other in the size and density of arrangement of the protein substance plates composing them (the yellow is smaller and denser), further the chalazae—two strands of protein extending to the edges of the egg and supporting it inside the shell in a suspended state, further two membranes, the so-called protein membranes, diverging at the blunt end of the egg and forming an air chamber there, and finally the calcareous shell of the egg. However, there is no lack of naked eggs either, especially in a number of lower invertebrates, where eggs capable of forming pseudopodia and amoeboid movement have been described in sponges and coelenterates. The presence of certain membranes, as well as the total number of eggs, is essentially related to the living conditions of the animal, the duration and environment of embryo development. In the presence of unfavorable development conditions (parasites, animals with external fertilization, etc.), the number of eggs increases significantly, and the number of protective devices of the egg ensuring its survival also increases; it is sufficient to refer to the thick egg membranes of parasites. Within the same species, in some cases, seasonal dimorphism of eggs is noted (winter and summer eggs), associated with the presence of fertilization and parthenogenesis.

Author signatures: A. Andreyev

S. Zalkind. The human egg. The question of the germ path (see Oogenesis) in humans cannot be considered resolved, although the data of G. Politzer on very early stages of development of human embryos (starting from 0.3 mm in length), in which he discovered primary sex cells (Urkeimzellen), speak in favor of its existence. The oogonia of the embryo first become morphologically distinct by the time of sex differentiation, i.e., at an age of about 2 months. They multiply intensively throughout the entire first half of intrauterine life; their multiplication ceases by 6-7 months. Already in the period of multiplication, other processes are observed in a number of oogonia. These latter proceed in three directions. Part of the oogonia transforms into oocytes of the first order, which is morphologically characterized by typical nuclear changes. In the nucleus of the oogonium, which has the appearance of a lightly stained and almost structureless vesicle with 1-3 nucleoli, chromatin accumulates in the form of lumps, clumps, and short threads, as a result of which the nucleus begins to stain darker. Such nuclei, according to von Winiwarter, already belong to first-order oocytes. Further, the entire nucleus appears filled with a tangle of thin convoluted threads that fold in pairs. The "bouquet," considered by some authors as characteristic of this stage, in the form of a dense tangle of threads pushed to one pole, is actually a vital artifact that easily arises due to the particularly high lability of nuclear structures at this stage. Another direction of changes in the sex cells is the degeneration of oogonia and especially oocytes. The vast majority of the latter perish in the embryonic ovary with phenomena of chromatopyknosis: the chromatin components of the nucleus shorten, while showing a tendency to pairwise arrangement. The karyoplasm swells, becoming more transparent and lighter. The general appearance of the nucleus of such degenerating cells resembles its structure during reduction division, which gave reason for erroneous interpretations of such pictures in this sense, but the similarity here is purely external. Maturation divisions do not occur in the embryonic ovary. The process of degeneration of the sex elements is maximally expressed in the ovary of 6-7 month fetuses. Finally, a certain part of the first-order oocytes, surrounded by a layer of follicular cells, forms a primordial follicle; by 7-8 months, all surviving sex cells are enclosed in them. If not one, but several oocytes fall into the composition of the primordial follicle, a polyovular primordial follicle arises. The egg cell undergoes its further development already inside the follicle. It has been studied still completely insufficiently, and especially little is known about nuclear changes during the maturation process itself. It is necessary to distinguish between the processes of maturation of the egg cell, for which both maturation divisions are decisive, proceeding apparently at the moment of ovulation or during the passage of the egg through the tubes (Allen, Bland, Newell and Pratt), and the maturation of the primordial follicle, transforming into a Graafian vesicle. The latter also, as a rule, takes place in the period of puberty, but as an exception can take place in the embryonic ovary, and the egg cell in such a case still remains a first-order oocyte. For mature egg see Ovulation, Embryo.

See S. Zalkind.