to grow, with varying velocities, by the addition of varying quantities of yeast. Here, as Roux is careful to point out6, we observe that it is not only the growth of the individual cells, but the traction exercised through their mutual interconnections, which brings about the foldings and other distortions of the entire structure. {56}
But this again was clearly present to Hallerãs mind, and formed an essential part of his embryological doctrine. For he has no sooner treated of incrementum, or celeritas incrementi, than he proceeds to deal with the contributory and complementary phenomena of expansion, traction (adtractio)7, and pressure, and the more subtle influences which he denominates vis derivationis et revulsionis8: these latter being the secondary and correlated effects on growth in one part, brought about, through such changes as are produced (for instance) in the circulation, by the growth of another.
Let us admit that, on the physiological side, Hallerãs or Hisãs methods of explanation carry us back but a little way; yet even this little way is something gained. Nevertheless, I can well remember the harsh criticism, and even contempt, which Hisãs doctrine met with, not merely on the ground that it was inadequate, but because such an explanation was deemed wholly inappropriate, and was utterly disavowed9. Hertwig, for instance, asserted that, in embryology, when we found one embryonic stage preceding another, the existence of the former was, for the embryologist, an all-sufficient ãcausal explanationã of the latter. ãWe consider (he says), that we are studying and explaining a causal relation when we have demonstrated that the gastrula arises by invagination of a blastosphere, or the neural canal by the infolding of a cell plate so as to constitute a tube10.ã For Hertwig, therefore, as {57} Roux remarks, the task of investigating a physical mechanism in embryology,ããder Ziel das Wirken zu erforschen,ããhas no existence at all. For Balfour also, as for Hertwig, the mechanical or physical aspect of organic development had little or no attraction. In one notable instance, Balfour himself adduced