Egg (biology)

“ But the egg itself must be considered as the original animal, as the infusorial creature, appearing in the globular or primary form of animal life, and, like the seed of the plant, containing ideally within itself the whole animal, which, under given external circumstances, it develops really out of itself. Recent observations have shown that the first rudiment of the embryo is formed on the inner surface of the hollow globe of the egg, by folding or turning inwardly a part of the integuments of the egg ”

“ In thus saying that the ova of all animals are, so far as microscopes can reveal, substantially similar, I am of course speaking of the egg-cell proper, and not of what is popularly known as the egg. The egg of a bird, for example, is the egg-cell, plus an enormous aggregation of nutritive material, an egg-shell, and sundry other structures suited to the subsequent development of the egg-cell when separated from the parent’s body. ”

“ What is common to all eggs and essential to them all is that which corresponds to the yolk in the bird's egg. But their later mode of development, the degree of perfection acquired by the egg and germ before being laid, the term required for the germ to come to maturity, as well as the frequency and regularity of the broods, are all features varying with the different kinds of animals. There are those that lay eggs once a year at a particular season and then die ”

“ In other words, when the egg is a mere mass of oil and albumen, not indicating as yet in any way the character of the future animal, and discernible only by the microscope, the distinction is indicated between the brains and the senses, between the organs of instinct and sensation and those of mere animal functions. At that stage of its existence, however, when the egg consists of an outer sac, an inner sac, and a dot, its resemblance to a cell is unmistakable; and, in fact, an egg, when forming, is nothing but a single cell. ”

“ The yolk of the egg is a still stronger or more nutritive fluid, which is drawn up into the bowels of the chick just at its exclusion from the shell, and serves it for nourishment for a day or two, till it is able to digest, and has learnt to choose the harder seeds or grains, which are to afford it sustenance. Nothing analogous to this yolk is found in the fetus of lactiferous animals, as the milk is another nutritive fluid ready prepared for the young progeny. ”

“ This assumption is supported by the fact that the amount of nourishment taken by the young female between the time of emergence from the egg and deposition of her first egg determines which kind of egg she shall subsequently produce. The well-nourished females produce female eggs, the poorly nourished produce male eggs. Sex, however, is determined while the ovum is still in the ovary. When the eggs have once been formed no subsequent change of food or temperature can alter the kind of eggs that are produced. ”

“ The protoplasm of all eggs contains nutritive material for the nourishment of the future embryo, and this material may be sufficient in quantity to make the whole cell, although remaining microscopic, conspicuously large, or to expand it to the relatively enormous mass of the yellow yolk of a fowl's egg. ”

“ The analogy between seeds and eggs has long been observed, and is confirmed by the mode of their production. The egg is known to be formed within the hen long before its impregnation; C.F. Wolf asserts that the yolk of the egg is nourished by the vessels of the mother, and that it has from those its arterial and venous branches, but that after impregnation these vessels gradually become impervious and obliterated, and that new ones are produced from the fetus and dispersed into the yolk. ”

“ In such a case, the egg itself divides into a number of portions: two, four, eight, or even twelve and sixteen individuals being normally developed from every egg, in consequence of this singular process of segmentation of the yolk,—which takes place, indeed, in all eggs, but in those which produce but one individual is only a stage in the natural growth of the yolk during its transformation into a young embryo. ”

“ We know, on the other hand, that when its growth is complete, after the formation of yolk and membranes, the egg contains germ-plasm, for it is capable of developing into an embryo. We have therefore, as it were, two natures in a single cell, which become manifest one after the other, and which, according to our fundamental conception, can only be explained by the presence of two different idioplasms, which control the egg-cell one after the other, and determine its processes of development. ”

“ I entitle conception the first mixture from the male and female (the analogue of the vegetable seed therefore) . Wherefore from one seed there is engendered one body, as from one egg one animal.”
It appears, consequently, that for one egg there is one soul or vital principle. [208] But whether is this that of the mother, or that{277} of the father, or a mixture of the two? And here the greatest difficulties are occasioned by those eggs that are produced by the concurrence of animals of different species, as, for example, of the common fowl and pheasant. ”

“ The typical fish-egg is globular, more or less transparent, having a tough protective membrane, within which the yolk-laden egg proper lies. The yolk is present in sufficient amount to maintain the embryo until it can swim about and feed for itself. ”

“ The largest eggs are those of birds; and this because, to the minute essential portion of the egg, or germ, from which the young bird grows, there is added a large store of food-material—the yolk and white of the egg—destined to nourish the growing embryo while the whole is enclosed within a hard shell. ”

“ Egg-cells, nutritive cells, and the cells of the follicle-epithelium (epithelium of the chambers of the ovarian tubes) are, says Lang, according to their origin, similar elements, like the egg and yolk-cells of the flat worms (Platodes) ; division of labor leads to their later differentiation. Only a few of the numerous egg-germs develop into eggs, the rest serving as envelopes and as food for these few. ”

“ The history of the egg shows a reversible condi­tion of rest and of activity. The primordial egg cell multiplies actively until a large number of eggs are formed in the ovary which may reach into the millions in the case of sea urchins or certain annelids. These cell divisions then stop and the egg goes into the resting stage in which it deposits the reserve material for the development of the embryo. From this condi­tion it can only be called into activity again by the spermato­zoön or the agencies of artificial parthenogenesis. ”

“ Either a lamp, candle, or lantern may be used on the inside of the box. Holding the egg in front of the box and letting the light shine through it will show the condition of the egg. An infertile egg will be clear, while a fertile egg has a spider-like center with threads leading out from it. All infertile eggs must be removed.
Poultry is classified as the egg breeds, the meat breeds, the general-purpose breeds, and the ornamental breeds.
The egg breeds are small fowls which mature quickly. They [Pg 87] are very active, but not good chickens to be used for setting purposes. ”

“ Such questions as whether the whole formative power in the egg is absolutely confined to one or two small centres to which the whole of the other egg contents are merely, as it were, passive accessories, or whether an egg is a combination in which some portion of the powers of rearrangement is possessed by other particles, as well as the chromosomes, in virtue of their own nature or of their position at an early period in the whole, can scarcely be settled without the aid of direct observation of the processes during life. ”

“ The eggs are laid in this mound and they are hatched by the heat of the sun and that produced by the fermentation of the grass, and they are never hatched by the parent birds. The egg is of a pale brown, but always stained by nesting materials. ”

“ It will require the aid of a microscope to discover and observe the object of our search and study. The egg of the bird bears some proportion to the size of its producer, for in them is stored up the nourishment which is to maintain the unfolding life that can be fed in no other way until it has reached an advanced stage of maturity, and has broken its shell and emerged into the outer world. The human egg is so small that it would require two hundred and forty of them laid side by side to make one inch in length. ”

“ These ligaments, according to Fabricius, is the part of the egg fecundated by the seminal spirit of the male; and it is here where the fœtus first begins to form. The egg is not only the true matrix, that is to say, the place of the formation of the chick, but it is from the egg all generation depends. The egg produces it as the agent: it supplies both the matter and the organs; the ligaments are the substance of formation; the white and the yolk are the nutriment, and the seminal spirit of the male is the efficient cause. ”

“ The egg-tubes above contain ripe eggs still surrounded by the follicle; e. c, the empty egg-chamber. Beyond the empty egg-chambers (e. c) are three egg-chambers with ripe eggs and the connecting cord. The whole tube is surrounded by the peritoneal membrane and musculature.—After Korschelt.
3. Comprising ovaries whose ends above the egg-germs contain a well-developed mass of cells functioning as a yolk-forming organ, between whose special elements grow root-like offshoots of nearly ripe egg-cells. (Hemiptera.) ”

“ It is worthy of notice that the eggs are shed like seeds, being dropped loosely and, as we have said, remaining on the ground or elsewhere, sometimes for nearly two years, without other protection than that they derive from their coverings. Each egg is really a capsule containing an egg, reminding us thus of the capsule of the Blattidae, which contains, however, always a number of eggs. ”

“ When they are ready to deposit, she is forced to place them whether she has mated or not. In case a mate has found her, a small pouch near the end of her abdomen is filled with a fluid that touches each egg in passing and renders it fertile. The eggs differ with species and are placed according to family characteristics. They may be pure white, pearl-coloured, grey, greenish, or yellow. There are round, flat, and oblong eggs. ”

“ The animal egg, which in its young state forms an element in the structure of the parent organism, possesses in the relations now under consideration a peculiar interest. The egg is a true cell, consisting essentially of a lump of protoplasm inclosing a nucleus, and having a nucleolus included in the interior of the nucleus. While still very young it has no constant form, and is perpetually changing its shape. Indeed, it is often impossible to distinguish it from the Amœba; and it may, like an Amœba, wander from place to place by the aid of its pseudopodial projections. ”

“ Another of the great steps in organic evolution was the differentiation of two different physiological types, the male or sperm-producer and the female or egg-producer. It seems to be a deep-seated difference in constitution, which leads one egg to develop into a male, and another, lying beside it in the nest, into a female. In the case of pigeons it seems almost certain, from the work of Professor Oscar Riddle, that there are two kinds of egg, a male-producing egg and a female-producing egg, which differ in their yolk-forming and other physiological characters. ”

“ The external covering of the egg, called by Pliny the cortex and putamen, by Quintus Serenus the testa ovi, is a hard but thin, friable and porous covering, of different colours in different cases—white, light green, speckled, &c. All eggs are not furnished with a shell on their extrusion: the eggs of serpents have none; and some fowls occasionally, though rarely, lay eggs that are without shells. ”

“ When eggs which have been submitted to incubation permit light to shine through and show the yolk suspended in the upper half of the center as a clearly defined mass, which quickly reassumes its position in turning the egg with its long axis nearly horizontal, they are probably infertile or sterile. ”

“ A rotten egg is distinguished from a fertile egg through the tester by the movement of the line between the transparent air space at the large end of the egg and the dark contents, this movement showing that the contents are in a fluid state. ”

“ But while we recognize the identity of cell-structure and egg-structure at this point in the history of the egg, we must not forget the great distinction between them,—namely, that, while the cells remain component parts of the whole body, the egg separates itself and assumes a distinct individual existence. ”

“ We usually think of eggs as consisting of a yolk surrounded by albumen, and enclosed in a hard shell made of lime or some form of calcium. The eggs of birds and fowls are our types, but the egg of the Earthworm has no chalk-like shell. ”

“ Development of the Egg in Incubation.—The development of the egg in incubation is very similar to the germination of a seed. An egg consists of albuminous and fatty matters, so inclosed that, while oxygen of the air is admitted, nutrient matters are excluded. During incubation the egg changes into an embryo; it passes from an almost unorganized to a highly-organized condition, from a lower to a higher condition. There is work done: there must be expenditure of force; but, as we have already seen, vital force is always derived from decomposition. ”