And this leads us to consider what biologists mean by an _organ_: it is a particular portion of the body set apart for the performance of some particular function. The whole process of development is this setting apart for special purposes. The starting-point of Life is a single cell--that is to say, a microscopic sac, filled with liquid and granules, and having within it a nucleus, or smaller sac. Paley has somewhere remarked, that in the early stages, there is no difference discernible between a frog and a philosopher. It is very true; truer than he conceived. In the earliest stage of all, both the Batrachian and the Philosopher are nothing but single cells; although the one cell will develop into an Aristotle or a Newton, and the other will get no higher than the cold, damp, croaking animal which boys will pelt, anatomists dissect, and Frenchmen eat. From the starting-point of a single cell, this is the course taken: the cell divides itself into two, the two become four, the four eight, and so on, till a mass of cells is formed, not unlike the shape of a mulberry. This mulberry-mass then becomes a sac, with double envelopes, or walls: the inner wall, turned towards the yelk, or food, becomes the _assimilating_ surface for the whole; the outer wall, turned towards the surrounding medium, becomes the surface which is to bring frog and philosopher into contact and relation with the external world--the Non-Ego, as the philosopher, in after life, will call it. Here we perceive the first grand “setting apart,” or _differentiation_, has taken place: the embryo having an assimilating surface, which has little to do with the external world; and a sensitive, contractile surface, which has little to do with the preparation and transport of food. The embryo is no longer a mass of similar cells; it is already become dissimilar, _different_, as respects its inner and outer envelope. But these envelopes are at present uniform; one part of each is exactly like the rest. Let us, therefore, follow the history of Development, and we shall find that the inner wall gradually becomes unlike itself in various parts; and that certain organs, constituting a very complex apparatus of Digestion, Secretion, and Excretion, are all one by one wrought out of it, by a series of metamorphoses, or _differentiations_. The inner wall thus passes from a simple assimilating surface to a complex apparatus serving the functions of vegetative life. Now glance at the outer wall: from it also various organs have gradually been wrought: it has developed into muscles, nerves, bones, organs of sense, and brain: all these from a simple homogeneous membrane!