layers still remain unaltered. At the commencement of the second day, the anterior portion of the embryo is dilated, and the three membranes which represent it have become bent down. At the conclusion of the second day, this inflection is carried still farther, and in the vascular layer a beating point, the punctum saliens, the first appearance of a heart, has become developed. On the third day, the serous membrane has become reflected over the back of the foetus; at one extremity investing the head with a serous covering, and at the other extremity investing the tail. This reflection of the serous membrane is finally to form the amnion or inner lining of the bag in which the fœtus is to be contained.

The mucous layer, or that next the yolk, at this time lines the open space which is to form the abdominal cavity, and by its inflections gives origin to the rudiments of the abdominal viscera.

The heart in the vascular layer is now seen to be composed of two chambers; and further, the branchial arteries are discovered which join to form the aorta.

EMBRYO OF BIRD.

With the vessels (i) of the vascular area, after four days' incubation.

On the fifth day, the outlines of the viscera are tolerably distinct, the sac of the amnion is completed, and the liver and lungs begin to appear; the bag of the altantois is well developed.

The heart is still that of

a fish, and the aorta formed by the branchial arches, which had been visible from the

e, Formation of the digestive cavity; c, embryo; f, layers of germinal membrane; h, heart; s, stomach.

third day.

The successive changes which take place on the vascular system are rather complex. Thus, of five pairs of vascular branchial arches, which at first by their union formed the aorta, as in fishes, those of the first pair on both sides, and of the fifth on the left side, speedily disappear. The third on each side becomes the brachio-cephalic trunks; the fourth of the right side becomes the descending aorta; while the fifth of the right side, and the fourth of the left side, are converted into the pulmonary arteries. The very short trunk common to the two pulmonary arteries, and also the equally short trunk of the aorta, are produced by the transformation of the single cavity of the original "bulbus arterosus" into two distinct canals; and thus this wonderful metamorphosis is completed.-The General Structure of the Animal Kingdom, by Jones, p. 627.

About five days from the commencement of incubation, the vascular layer of the germinal membrane has spread extensively over the yolk; and as the vessels are formed, they are found to converge towards the navel of the embryo, and to constitute a distinct system of arteries and veins communicating with the aorta and the heart of the fœtus, and forming a vascular circle surrounding the yolk. These vessels are termed

omphalo-mesenteric vessels. The omphalo-mesenteric arteries arise from the mesenteric arteries, and the omphalo-mesenteric veins return to the vena cava of the chick.

When the intestinal system has reached some degree of development, a communication is found to have arisen between the yolk and the intestine, by a wide duct termed the vitello-intestinal duct, and by which the nutritive substance of the yolk enters the alimentary canal for the alimentation of the embryo. By the time incubation is completed, the yolk-bag is empty, and the place of the duct is marked merely by a little cæcal appendage.

The allantoid membrane first makes its appearance in the early part of incubation, while the abdomen is still open, as a delicate bag derived from the anterior part of the rectum; but it quickly enlarges, so as at last to line nearly the whole extent of the membrane of the shell; and being thus exposed to the air, which penetrates the shell,

9

FORMATION OF THE ALLANTOIS.

c, e, embryo; g, layers of germinal membrane; s, stomach; i, allantois.

it becomes an important organ of respiration. When fully developed, it is copiously supplied with arteries and veins. The arteries derived from the common iliac trunks correspond to the umbilical arteries in mammals, and the veins corresponding to the umbilical veins, reach the inferior vena

cava.

About the nineteenth day of incubation, the air-vessel at the large extremity of the egg is ruptured, the lungs begin to breathe the air which it contained, and the vessels of the allantois become by degrees obliterated. On the twenty-first day the chick escapes from the shell, to begin a new phase of life.

On the fourth day the chick is about four lines in length; on the sixth day it is seven lines; and then what appear to be voluntary motions are first observed. Ossification commences on the ninth day, and on the fourteenth day the feathers appear; and if taken out of the egg, the chick can open its mouth.

Reproduction in the Vegetable Kingdom.-The first stage of reproduction in the vegetable kingdom is the maturation of the seed; the second stage the germination of the seed, by which a new living plant is produced.

The seed is matured, as a general rule, within the inferior portion of the pistil or female organ, termed sometimes germen, sometimes ovary-the latter term being most used. This inferior portion of the pistil becomes, by maturation, the fruit or seedvessel, called also the pericarp. Familiar examples of the pericarp are the cherry, the apple, the pear, the poppy-head, the flat pouches of garden honesty, and of shepherd's purse, the French bean, and the pea-pod.

If we examine the several flowers in which these pericarps form, we shall find all of them are mere enlargements of the base of the pistil-that is, of the germen or ovary.

If the interior of the germen or ovary be examined at an early period, the rudiments of the seeds are found to be already present in the form of minute membranes not yet closed in on every side. The condition requisite for the perfect development of these rudimentary parts into perfect seeds is the entrance into their interior of a pollen granule derived from the male organ or stamen. The upper part of the stamen, named

anther, secretes the pollen, which, being transferred to the upper part of the pistil, there finds entrance, and descends through the middle part or stile into the cavity of the germen or ovary, and finally into the cavity of the rudiment of the seed. Forthwith the seed becomes developed into a perfect part.

The perfect seed contains, beneath its exterior membranes, a part destined to be developed into the stem, another part destined to be developed into the root, and other parts destined to supply nourishment up to the period when the new individual has attained sufficient development to draw the means of support from the soil and from the atmosphere. The nourishment contained in the substance of seeds is starch.

The conditions necessary for the development of a seed into a new plant are the presence of moisture, warmth, and atmospheric air. When put into the earth, not far from the surface, the seed swells by the agency of moisture, and imbibes oxygen from the air diffused through the water of the soil. In proportion as it acquires oxygen, it throws off carbonic acid. The starch during this process is, in part at least, changed to sugar, or to a soluble substance more readily conveyed onwards, as the stem and radicle are developed. Besides starch, the seed contains certain saline bodies, such as phosphates, and the other mineral constituents found in organic bodies, which serve for a supply till the root is sufficiently developed to draw such constituents from the soil.

Under the influences of these sources of supply, the gemmule, or part of the seed representing the stem, at last arises above the ground, and the radicle, or part representing the root, descends into the earth. The parts of the seed destined to supply nourishment are the seed lobes, or cotyledonary bodies, and the albumen; the latter being present only in certain orders of seeds. When the albumen is absent, the cotyledonary bodies are proportionally larger. In many plants these seed-lobes, or cotyledonary bodies, rise above ground in the form of temporary leaves, and plainly perform for a time the office of leaves, by drawing nourishment from the atmosphere. But as the proper leaves form on the stem, the cotyledonary bodies, whether they ascend into the atmosphere or remain below ground, shrivel and decay; and the same thing happens to the albumen when it is present.

When this stage is attained, the growth of the new individual proceeds on much the same plan as in mature plants.

Recapitulation.-Such, then, is an outline of organic life in the two great departments of nature endowed with vitality; and a brief review of the connexions of these two kingdoms of nature with each other, and of their common dependence on mineral nature, will form a proper conclusion to this section of our treatise.

We have seen that the elements which compose the animal kingdom exist in the mineral kingdom. The original position of these elements is in the rocks composing the crust of the earth, and in the water which rests on its surface, or in its gaseous envelope, the atmosphere. The next position in which these elements are found, previously to their becoming part of the substance of the bodies of animals, is in the component parts of the vegetable kingdom-namely, in parts of vegetables which serve for food to animals, such as the roots of the potato, the turnip, the carrot, the parsnip, the onion, the leek, the beet, the leaves of the various species of brassica, spinach, parsley, lettuce, the seeds of wheat, barley, oats, and Indian corn, rice, and the like. We next find these elements advanced to the rank of constituents of an animal body, and sometimes passing from one animal body to another. The next transition of these elements

86

RELATION OF ORGANIC TO MINERAL NATURE.

is a return to the mineral state; not, indeed, for the most part to resume their original form, if that were the component parts of a rock, but to enter into the soil, or to join the waters of the surface, or to float in the air till received again into the vegetable kingdom, to perform the same round as before.

For example, one of the ores of manganese, which when exposed to heat gives off oxygen abundantly, occurs in the oldest strata of the crust of the earth. Thus, an atom of oxygen which has lain fixed in a rock for an incalculable number of ages, may have been set free only a year or two ago, and yet if the history of its progress could be traced, it would fill a volume.

Its first condition, after being set free from its imprisonment, is a particle freely floating in the atmosphere. We may suppose, then, that it descends to the earth absorbed in a drop of rain. It unites with a minute portion of carbon existing in the soil, to form carbonic acid, which, being taken up by the root of some useless weed, is conveyed to a leaf, and then again set free,—its companion, the carbon, being retained. We may next suppose that amid a thunder-storm, as it floats high in the air, it is yoked to an atom of hydrogen, to form an atom of water, and that it again descends to the earth; now not as an impregnation but as a minute integral portion of a drop of rain. It is again taken up, we will suppose, by the radicle of such a grass as the common poa or meadow grass, and the atom of water being decomposed, it becomes fixed in a minute portion of albumen within the leaf of the grass. By-and-by this grass is cropped by a cow grazing in the pasture; and the albumen being soon changed to caseine, it comes forth as a constituent of milk. It is quickly found in a human stomach undergoing the process of digestion, and being received into the blood circulates there, to escape, perhaps, from its new possessor by a cut of the finger.

The blood left exposed to the air quickly putrefies, and our atom of oxygen escapes from the fibrine or albumen in which it existed, in company again with carbon, or in the form of carbonic acid. It probably soon comes into contact with a leaf, for example a spinach leaf, and the carbon being disjoined from it and fixed in the plant, our atom again becomes free. It now, for the first time, becomes the victim of respiration, being drawn into the lungs of a passer-by. Being conveyed over his body with the arterial blood, after passing through his heart, it is quickly found uniting with the debris of: the muscular fibres which have been longest in action; and, returning in the venous blood to the lung, united with a portion of carbon, is thrown out as a part of the expired air, in the shape of carbonic acid. It is now carried high into the air, and falling into the southward current, is quickly found journeying westward, with the trade-wind, at a lower level and in a warmer region. As it reaches the luxuriant vegetation of a West Indian Island, it is speedily disjoined from its associated carbon, and again set free, leaving its companion to form part of the substance of a luxuriant banana. Soaring again in the air, it forms part of the northern current, and in no long time is again found fit to assist the respiration of the inhabitants of the same region from which a short time before it had departed in company with a particle of carbon.

Such is a slight specimen of the unceasing changes which the particles composing organic nature undergo. There is a circulation of particles from the mineral kingdom through the vegetable to the animal kingdom; and the air which the animal kingdom contaminates the vegetable kingdom purifies. Lastly, the surplus of contaminated air, which the limited vegetation of temperate countries cannot purify, is wafted to feel the influence of a tropical vegetation, and brought back restored to the required state of purity for animal existence.

With the following passage from Liebig, one of the greatest names in organic chemistry, we shall close our account of the vegetative functions:-"When the vegetable kingdom, in the temperate and cold zones, ceases to decompose the carbonic acid generated by the processes of respiration and combustion, the proper, constant, and inexhaustible sources of oxygen gas are the tropics and warm climates, where a sky, seldom clouded, permits the glowing rays of the sun to shine upon an immeasurably luxuriant vegetation. In our winter, when artificial warmth must replace deficient heat of the sun, carbonic acid is produced in superabundance, and is expended in the nourishment of tropical plants. The great stream of air which is occasioned by the heating of the equatorial regions, and by the revolution of the earth, carries with it, in its passage to the equator, the carbonic acid generated during our winters; and in its return to the polar regions brings with it the oxygen produced by the tropical vegetation."

The Locomotion of Animals.-The next subject for our consideration is that function by which living beings are enabled to move from place to place. Among the lowest orders of animal existences, as in some zoophytes and mollusca, we find those which are permanently stationary, and, like plants, unable to leave the substance to which they are attached. And even some of these who do move about, as the seablubber, the sea-pen, and many others, do so passively; and, like the duckweed and star-grass among plants, are moved in water chiefly by the currents, and tides, and winds; but the number of those in whom locomotion is otherwise than active, is certainly very small. Again, during one period of their existence, the fixed zoophytes do possess a power of locomotion. Thus the young sponge, after its separation from the parent stem, for several days swims about as if to find the appropriate spot to which it may attach itself; while the cilia, or arm-like appendages, to the action of which its locomotive powers are due, fade and disappear, as if no longer required, after the animal has attached itself to the rock. Similar properties are found among the polypes lodged in the madrepores and corals, with which all are familiar. In the hydra, a species of polype inhabiting our fresh waters, for the knowledge of which we are indebted to M. Trembley, of Geneva, we find an early example of locomotive powers curious in the extreme. If the animal is introduced into a glass, it may

LOCOMOTION OF HYDRA VIRIDIS-after Trembley.

be seen, as in the figure, when standing erect slowly to bend its body, until its mouth touches the surface of the vessel; its foot is then detached, and brought towards the head, which is then projected forwards, and the process repeated, until a desirable position is obtained. We will pass over the Infusoria, so named by being to be found in all animal or vegetable infusions, after being kept a sufficient time; since they are all microscopic, and not to be seen by the naked eye. Their movements are very rapid; and the microscope reveals, as is familiar to most persons, a strange and busily moving mob even in a drop of water. Among the Medusa some are remarkable for their organs of locomotion, being furnished with an apparatus not unlike the fins of a fish,