[I] In some forms of life the opening of the cup marks the position of the future mouth: in others, of the future vent. In yet others it elongates into a slit, occupying the whole length of the embryo; the middle part of the slit closes up, and the opening at the far ends mark the position, the one of the future mouth, the other of the future vent. [J] In technical language, the outer layer of cells is called the _epiblast_, the inner layer the _hypoblast_, and the mid-layer between them the _mesoblast_. [K] In technical language, the opening by which the primitive digestive cavity (or _mesenteron_) communicates with the exterior is called the _blastopore_. When this closes, the new opening for the mouth is called the _stomod[oe]um_; that for the vent, the _proctod[oe]um_. [L] We have seen that when volume tends to outrun surface, fission may take place, whereby the same volume has increased surface. But in unfavourable nutritive conditions, the same surface which had before been sufficient for nutrition may become, under the less favourable circumstances, insufficient, and fission may again take place to give a larger absorbent surface. Hence, possibly, the connection between insufficient nutriment and highly subdivided sperms. CHAPTER IV. VARIATION AND NATURAL SELECTION. Everything, so far as in it lies, said Benedict Spinoza, tends to persist in its own being. This is _the law of persistence_. It forms the basis of Newton's First Law of Motion, which enunciates that, if a body be at rest, it will remain so unless acted on by some external force; or, if it be in motion, it will continue to move in the same straight line and at a uniform velocity unless it is acted on by some external force. Practically every known body is thus affected by external forces; but the law of persistence is not thereby disproved. It only states what would happen under certain exceptional or perhaps impossible circumstances. To those ignorant of scientific procedure, it seems unsatisfactory, if not ridiculous, to formulate laws of things, not as they are, but as they might be. Many well-meaning but not very well-informed people thus wholly misunderstand and mistake the value of certain laws of political economy, because in those laws (which are generalized statements of fact under narrowed and rigid conditions, and do not pretend to be inculcated as rules of conduct) benevolence, sentiment, even moral and religious duty, are intentionally excluded. These laws state that men, under motives arising out of the pursuit of wealth, will act in such and such a way, unless benevolence, sentiment, duty, or some other motive, lead them to act otherwise. Such laws, which hold good, not for phenomena in their entirety, but for certain isolated groups of facts under narrowed conditions, are called _laws of the factors_ of phenomena.