Yolk sac

“ Naturally, in birds where the yolk-sac is of great functional importance this diverticulum is large, and in a majority of the families of birds persists throughout life, forming a convenient point of orientation. In mammals, no doubt in association with the functional reduction of the yolk-sac, this diverticulum, which is known as Meckel’s diverticulum, has less importance, and whilst it has been observed in a small percentage of adult human subjects has not been recognized in the adult condition of any lower Mammalia. ”

“ The mid gut forms a loop with its convexity toward the diminishing vitelline duct, or remains of the yolk-sac, and until the third month it protrudes into the umbilical cord. The greater curvature of the stomach grows more rapidly than the lesser, and the whole stomach turns over and becomes bent at right angles, so that what was its left surface becomes ventral. ”

“ In the region of the yolk-sack proper the blastoderm is so thin that it is not easy to be quite sure that a layer of hypoblast is throughout distinct. Both the hypoblast and mesoblast of the yolk-sack are formed by a differentiation of the primitive lower layer cells.
Nutriment from the yolk-sack is brought to the embryo partly through the umbilical canal and so into the intestine, and partly by means of blood-vessels in the mesoblast of the sack. The blood-vessels arise before the blastoderm has completely covered the yolk. ”

“ The eggs, which line the periphery of the sac, develop into embryos which presently become larvae, that is to say, instead of further development at the expense of yolk-cells contained 435within themselves, they develop a temporary stomach and a large sucking organ, and become for a time independent sucking animals, imbibing the fluids in the common sac, and arranged around its circumference with their mouths directed towards the centre. ”

“ Some, which were just hatched, were less than three-quarters of an inch long, and scarcely able to move about in the water because of the great weight of the yolk about the center of their bodies. A few had consumed a large part of the sac.
"It'll take them about six weeks to get rid of the yolk," the foreman said, anticipating the boy's question, "and if they were in a natural stream [Pg 143] they would be able to look after themselves. ”

“ The whole body soon becomes flexed ventrally, and also somewhat spirally. The yolk-sack (b) forms a small spherical appendage with a long wide stalk, and the embryo (B) is attached by an allantoic stalk with a slight swelling (all) , probably indicating the presence of a small hypoblastic diverticulum, to the inner face of the chorion. ”

“ The space between the inner and outer sack is, from the mode of its formation, simply a part of the general cavity found everywhere between somatopleure and splanchnopleure. The outer sack over the embryo lies close under the vitelline membrane, and the cavity between it and the true amnion is gradually extended over the whole yolk-sack. ”

“ Although most Anurous embryos are not provided with a sufficient amount of yolk to give rise to a yolk-sack as an external appendage of the embryo, yet in some forms a yolk-sack, nearly as large as that of Teleostei, is developed. One of these forms, Alytes obstetricans, belongs to a well-known European genus allied to Pelobates. The embryos of Pipa dorsigera (Parker) are also provided with a very large yolk-sack, round which they are coiled like a Teleostean embryo. A large yolk-sack is also developed in the embryo of Pseudophryne australis. ”

“ The yolk-sack is persistent, and fuses with the non-allantoic part of the subzonal membrane; which is rendered vascular by its blood-vessels. There would seem to be (Owen) a small decidua reflexa. A similar arrangement is found in the Hedgehog (Erinaceus Europæus) (Rolleston) , in which the placenta occupies the typical dorsal position. It is not clear from Rolleston’s description whether the yolk-sack persists till the close of fœtal life, but it seems probable that it does so. There is a considerable reflexa which does not, [Pg 244] however, cover the whole chorion. ”

“ If you can buy it or are willing to make it raw milk yogurt containing lactobacillus bulgaris or acidophilus may be digested more readily, especially if it prepared from healthy cows or goats fed on unsprayed food, and served very fresh. Eggs should come from chickens that run around outside, eating weeds, and scratching bugs. The yokes of those eggs will be intense orange, not yellow. Few people these days have ever eaten a real egg. Surprisingly, for those of you who fear cholesterol, the healthy way to eat eggs is use just the raw yolk from fertile eggs. ”

“ If you break an egg the contents flow out and are seen to consist of the colorless glairy “white” and the yellow “yolk.” If the white is collected by itself in water and then heated it becomes turbid, forming a white solid, very similar to the vegetable albumin, which is called animal albumin.
If the yolk is beaten up with water, no starch nor cellulose is obtained from it, but there will be plenty of fatty and some saccharine matter, besides substances more or less similar to albumin and gluten. ”

“ The protein of the yolk is chiefly in the form of ovovitellin, while the fats occur as palmitin, olein, and stearin. There is also 5% of coloring matter in the yolk of eggs besides lecithin, nuclein, salts of iron, potassium, magnesium, and phosphorus. The latter mineral salt comprises 1.0% in yolk, while in the white there is only .03%. Eggs have a position in the invalid dietary second only to that of milk. They are nutritious, easy of digestion, and exceedingly palatable if properly selected and correctly prepared. The albumen in the white is very susceptible to the effect of heat. ”

“ By degrees this fluid is transformed into the small particles or granules which form the yolk; the yolk shapes itself into a multitude of cells--little microscopic bodies consisting of an external membrane, or cell-wall, and of an inner nucleus, which may be either solid or fluid; and in due process of time a number of cells combine and form a living being. The albumen, or "white," is, like the shell, an accessory. It performs important functions in the development of the young from the germ, but we will not stop to explain them here; the true egg is the yolk. ”

“ The little creature which has just pecked its way out of its calcareous prison-house, has lime and phosphorus in his bones, sulphur in his feathers, iron, potash, soda and magnesia in his blood, all of which mineral constituents come from the egg, and are taken into the stomach when it is eaten as food.
The valuable or important salts are contained in the yolk, and hence this portion of the egg is the most useful in some forms of disease. A weakly person, in whom nerve force is deficient and the blood impoverished, may take the yolks of eggs with advantage. ”

“ Eggs, when not fresh, are more or less liable to disease. I might even say more. When not fresh, they are diseased. On this point we have the testimony of Drs. Willich and Dunglison. The truth is, that the yolk of the egg has a strong tendency to decomposition, and this decomposing or putrefying process begins long before it is perceived, or even suspected, by most people. There is much reason for believing that a large proportion of the eggs eaten in civic life,—except when we keep the poultry ourselves,—are, when used, more or less in a state of decomposition. ”

“ The organ to which these are attached is the ovary. The smallest yolks are so small that they cannot be seen without a powerful microscope. These yolks are not germs, but as they grow the germ forms on one side of each yolk, where it appears as a small white spot.
When a yolk is full-grown it drops into the funnel-shaped mouth of the oviduct. Here it is inclosed in a membranous covering, called the chalazæ, and receives a coating of thick albumen. The function of the chalazæ is to keep the yolk suspended in the center of the egg. ”

“ 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. ”

“ On the whole I have found osmic acid the most suitable reagent for the study of the yolk, since without breaking up the developing spherules, it stains them of a deep black colour. The yolk spherules commence to be formed in ova, of not more than 0.06 mm. in the ovaries of moderately old females. In young females they are apparently not formed in such small ova. They arise as extremely minute, highly refracting particles, in a stratum of protoplasm some little way below the surface, and are always most numerous at the pole opposite the germinal vesicle. ”

“ If the yolk be removed and dried, and the yellow oil separated, it will be found to form two-thirds of the substance. The whole weight in its natural state is about three hundred grains, of which three-fifths is water; of the white more than three quarters is water.
The yolk and albumen of a fecundated egg remains as sweet and free from corruption during the whole time of incubation as it is in new-laid eggs, and there is but little loss of water; whereas an unfecundated egg passes rapidly into putrefactive decay and perishes. ”

“ Let it be understood that eggs may lose their nourishment by cooking. The yolk, raw or very slightly boiled, is exceedingly nutritious. It is, moreover, the only food for those afflicted with jaundice. When an egg has been exposed to a long continuance of culinary heat, its nature [Pg 36] is entirely changed. A slightly-boiled egg, however, is more easy of digestion than a raw one. The best accompaniment for a hard egg is vinegar. Raw eggs have a laxative effect ”

“ The white of eggs, almost pure albumin, is nutritious, and, when cooked in water at 170 degrees Fahrenheit, requires less time for perfect digestion than a raw egg. The white of a hard-boiled egg is tough and quite insoluble. The yolk, however, if the boiling has been done carefully for twenty minutes, is mealy and easily digested. Fried eggs, no matter what fat is used, are hard, tough and insoluble. ”

“ Nearly all the Platyhelminthes are HERMAPHRODITIC, and in nearly all there are, in addition to the ovaries producing ova, other glands attached to the female genital apparatus, namely, the vitellaria or yolk glands, which provide a substance termed yolk, which serves as nourishment for the embryo. The fully formed eggs have shells and are “compound,” i.e., composed of the egg or ovarian cell, which is surrounded by numerous yolk cells or their products of disintegration. ”

“ Most persons are familiar with the appearance of eggs, but in order that satisfactory results may be obtained in their selection, care, and cooking, it will be necessary to look into the details of their composition. As is well known, an egg consists of a porous shell lined with a fine, but tough, membrane that encloses the white and the yolk and serves to protect them. The yolk is divided from the white by a delicate membrane, which permits it to be separated from the white when an egg is carefully broken. ”

“ The fat is accumulated in the yolk, often amounting to thirty per cent. Raw and lightly-boiled eggs are easy of digestion, but hard-boiled ones decidedly not so. An egg loses its freshness within a day or so. The shell is porous; and the always-feeding and destroying oxygen of the air quickly gains admission, causing a gradual decomposition. To preserve them, they must be coated with lard or gum, or packed in either salt or oats, points down. In this way they keep good a long time, and while hardly desirable to eat as boiled eggs, answer for many purposes in cooking. ”

“ The whole egg is generally used. The white alone will serve, but not the yolk alone, as it is the albumen which is needed. The albumen quickly coagulates when put into the hot fat, and forms a coating through which the grease will not penetrate. To one egg is added one tablespoonful of water, so as to make it thin enough to run and remove the stringiness of the egg; these are beaten lightly together, but should not be foamy, as bubbles break and leave holes for the grease to enter. Where delicate color is wanted, it is better to use the white of the egg only and fresh crumbs. ”

“ 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 lower or hypoblast segments are larger, in some cases very much larger, than those of the epiblast, and are so granular that their nuclei can only with difficulty be seen. They form a somewhat irregular mass, several layers deep, and thicker at the periphery than at the centre: they rest on a bed of white yolk, from which they are in parts separated by a more or less developed cavity, which is probably filled with fluid yolk matter about to be absorbed. In the bed of white yolk nuclei are present, which are of the same character, and have the same general fate, as those in Elasmobranchii. ”

“ An egg-cell consists of living protoplasm and a nucleus, a variable quantity of non-living food-yolk, and of certain enveloping and protective egg-membranes. The ova of Fishes differ principally in the amount and disposition of the food-yolk, in the character of the egg-membranes, and in the presence or absence of special perforations in the egg-membranes for the entrance of spermatozoa into the eggs. ”