We shall try to sketch the outlines of what might properly be called an *analytical theory of morphogenesis*; that is, to explain the sum of our knowledge about organic form-production, gained by experiment and by logical analysis, in the form of a real system, in which each part will be, or at least will try to be, in its proper place and in relation with every other part. Our analytical work will give us ample opportunity of mentioning many important topics of so-called general physiology also, irrespective of morphogenesis as such. But morphogenesis is always to be the centre and starting-point of our analysis. As I myself approach the subject as a zoologist, animal morphogenesis, as before, will be the principal subject of what is to follow. 2. ANALYTICAL THEORY OF MORPHOGENESIS[24] [24] Compare my *Analytische Theorie der organischen Entwickelung*, Leipzig, 1894, and my reviews in *Ergebnisse der Anatomie und Entwickelungsgeschichte*, vols. viii. xi. xiv., 1899-1905. A shorter review is given in *Ergebnisse der Physiologie*, vol. v., 1906. The full literature will be found in these reviews. α. THE DISTRIBUTION OF MORPHOGENETIC POTENCIES *Prospective Value and Prospective Potency* Wilhelm Roux did not fail to see that the questions of the locality and the time of all morphogenetic differentiations had to be solved first, before any problem of causality proper could be attacked. From this point of view he carried out his fundamental experiments. It is only in terminology that we differ from his views, if we prefer to call our introductory chapter an analysis of the distribution of morphogenetic potencies. The result will be of course rather different from what Roux expected it would be. Let us begin by laying down two fundamental concepts. Suppose we have here a definite embryo in a definite state of development, say a blastula, or a gastrula, or some sort of larva, then we are entitled to study any special element of any special elementary organ of this germ with respect to what is actually to develop out of this very element in the future actual course of this development, whether it be undisturbed or disturbed in any way; it is, so to say, the actual, *the real fate* of our element, that we take in account. I have proposed to call this real fate of each embryonic part in this very definite line of morphogenesis its *prospective value* (“prospective Bedeutung” in German). The fundamental question of the first chapter of our analytical theory of development may now be stated as follows: Is the prospective value of each part of any state of the morphogenetic line constant, *i.e.* is it unchangeable, can it be nothing but one; or is it variable, may it change according to different circumstances?