At a later stage of the investigation, we see why the Purkinjean vesicle, or inner sac of the egg, is placed on the side, instead of being at the centre, as in the cell.  It arises on that side along which the axis of the little Turtle is to lie,—­the opposite side being that corresponding to the lower part of the body.  Thus the lighter, more delicate part of the substance of the egg is collected where the upper cavity of the animal, inclosing the nervous system and brain, is to be, while the heavy oily part remains beneath, where the lower cavity, inclosing all the organs of mere material animal existence, is afterwards developed.  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.  This comparison is important, because there are both animals and plants which, during their whole existence, consist of a single organic cell, while others are made up of countless millions of such cells.  Between these two extremes we have all degrees, from the innumerable cells that build up the body of the highest Vertebrate to the single-celled Worm, and from the myriad cells of the Oak to the single-celled Alga.

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.  Even now, while still microscopically small, its individuality begins; other substances collect around it, are absorbed into it, nourish it, serve it.  Every being is a centre about which many other things cluster and converge, and which has the power to assimilate to itself the necessary elements of its life.  Every egg is already such a centre, differing from the cells that surround it by no material elements, but by the principle of life in which its individuality consists, which is to make it a new being, instead of a fellow-cell with those that build up the body of the parent animal and remain component parts of it.  This intangible something is the subtile element that eludes our closest analysis; it is the germ of the immaterial principle according to which the new being is to develop.  The physical germ we see; the spiritual germ we cannot see, though we may trace its action on the material elements through which it is expressed.