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Show 18 THE PRESENT CONDITION with one another, and from that conjunction, from that union which then takes place, there results the forma-ti. o n of a new bel·nOo' · At stated times the mare, from a parti.C u 1a r pat· t of the interior of her body, called the ovary, gets rid of a minute particle of .matter com parable in all essential respects with that w luch we called a cell .a little while since, which cell contains a kind of nucleus in its centre, surrounded by a clear space and by a viscid mass of protein substance (Fig. 2) ; ancl though it is different in appearance from the eggs which we are mostly acquainted with, it is really au egg. After. a time this minute particle of matter, which may only be a small fraction of a grain in weight, undergoes a series .0f changes,-wonderful, complex changes. Fi~ally, u~on its surface there is fashioned a little elevatwn, w h1Ch afterwards becomes divided and marked by a groove. The lateral boundaries of the groove extend upwards and downwards, ancl at length give rise to a double tnbe. In the upper snu1.ller tube the spinal marrow ancl brain are fashioned; in the lower, the alimentary ·Canal and heart, and at length two pairs of buds shoot out at the sides of the body, which are the rudiments of the limbs. In fact a true drawing of a section of the embryo in this state wouhl in aU essential respects resemble that diagran1 of a horse reduced to its simplest expression, which I first placed before you (Fig. 1). Slowly and gradually these changes take place. rrhe whole of the body, at first, can be broken up into "cells," which becom~ in one place metamorphosed into muscle, -in another place into gristle and bone,-in another place into fibrous tissue,-and in another into .hair; e·very part becoming gradually and slowly fashioned, as Ol!, ORGANIC NATURE. 10 if there were an artificer at work in each of these complex structures that we have 1nentioned. This embryo, as i.t is called, then passes into other conditions. This diagram represents the embryo of a dog ; and I should tell you that there is a time when the embryos of neither dog, nor horse, nor porpoise, nor monkey, nor man, can be distinguished by any essential feature one from the other; there is a time when they each and all of them resemble this one of the Dog. But as development advances, all the parts acquire their speciality, till at length you have the embryo converted into the form of the parent from which it started. So that you see, this living animal, this horse, begins its existence as a minute particle of introgenous matter, which, being supplied with nutriment (derived, as I haYe shown, from the inorganic world), grows up according to the special type and construction of its parents, works and undergoes a constant waste, and that waste is made good by nutriment derived from the inorgani€ world; the waste given off in this way being directly added to the inorganic world; and eventually the animal itself dies, and, by the process of de com position, its whole body is returned to those conditions of inorganic matter in which its substance ongtnated. This, then, is that which is true of every living form, from the lowest plant to the highest animal-to man himself. You might define the life of every one in exactly the same terms as those which I have now used; the difference between the highest and the lowest being simply in the complexity of the developmental changes, the variety of the structural forms,,. |