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 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!

With this bird's-eye view of the course of Development, you will be able to appreciate the grand law first clearly enunciated by Goethe and Von Baer, as the law of animal life, namely, that Development is always from the general to the special, from the simple to the complex, from the homogeneous to the heterogeneous; and this by a gradual series of differentiations. Or to put it into the music of our deeply meditative Tennyson:—

"All nature widens upward. Evermore The simpler essence lower lies: More complex is more perfect—owning more  Discourse, more widely wise."

You are now familiarized with the words "differentiation" and "development," so often met with in modern writers; and have gained a distinct idea of what an "organ" is; so that on hearing of an animal without organs, you will at once conclude that in such an animal there has been no setting apart of any portion of the body for special purposes, but that all parts serve all purposes indiscriminately. Here is our Opalina, for example, without mouth, or stomach, or any other organ. It is an assimilating surface in every part; in every part a breathing, sensitive surface. Living on liquid food, it does not need a mouth to seize, or a stomach to digest, such food. The liquid, or gas, passes through the Opalina's delicate skin, by a process which is called endosmosis; it there serves as food; and the refuse passes out again by a similar process, called exosmosis. This is the way in which many animals and all plants are nourished. The cell at the end of a rootlet, which the plant sends burrowing through the earth, has no mouth to seize, no open pores to