[60] This statement is *not strictly* correct for *Tubularia*. I found (*Archiv f. Entwickelungsmechanik*, ix. 1899), that a reduction of the length of the stem is always followed by a reduction of the size of the hydranth-primordium, but there is no real proportionality between them. It is only for theoretical simplification that a strict proportionality is assumed here, both in the text and the diagram. But there is an almost strict proportionality in all cases of “closed forms.” [Illustration: Fig. 12.--Tubularia. *a.* Diagram of the “Hydranth,” with its short and long tentacles. *b.* Restitution of a new hydranth inside the perisarc (*p*). *c.* The same--later stage; the tentacles are complete; the whole hydranth will be driven out of the perisarc by a process of growth that occurs at the locality marked ⬆. *d.* A stem of *Tubularia* cut either at *a_1 b_1* or at *a_2 b_2*, or at *a_1 c*. *e.* Position of tentacles in the piece cut at *a_1 b_1*. *f.* Position of tentacles in the piece cut at *a_2 b_2*, which is equal in length to *a_1 b_1*. *g.* Position of tentacles in the piece cut at *a_1 c*, which is half as long as *a_1 b_1*.] So we find our formula *p.v. (X) = f(s, l, E)* very well illustrated in *Tubularia*. The formula indeed may help us to predict, in any case, where a certain part of the polyp’s organisation is to originate, at least if we know all that is included under our letter *E*, *i.e.* the normal proportion of our form. Of course such prediction would not have much practical importance in all our cases of morphogenesis, but nevertheless I should like to state here that it is possible; for many scientific authors of recent times have urged the opinion that prediction of, and domination over, what will happen, can be the only true aims of sciences at all. I myself judge these aims to be of second or third-rate importance only, but, if they may be reached by what our purely theoretical study teaches, so much the better.