Ziegler’s results show very clearly that we are not obliged to think of the substance of the micromeres as laid down in the protoplasm of the egg, and hence there is no ground for supposing the substance of the paddles is _necessarily_ present in the vegetative hemisphere of the egg. His results show that if the vegetative part is cut off, micromeres and paddles are still formed, although that part of the egg substance from which they normally arise has been removed. It should be pointed out, in this connection, that Driesch and Morgan did not suppose that the bases of the micromeres, or of the paddles, are actually laid down in a definite part of the protoplasm of the egg. They had also observed that in some cases whole embryos arose after a part of the egg had been removed, and this they attributed to the symmetrical position of the cut in relation to the organization of the egg. Ziegler’s operations were made more or less in this symmetrical plane, excepting the one that gave rise to an incomplete embryo. Driesch and Morgan held that the formative factors become localized in the protoplasm, rather than arise from the nucleus, but pointed out that these observations do not lead to His’s conclusion of localized germ areas in the egg. CHAPTER XII THEORIES OF DEVELOPMENT The experimental work that Pflüger carried out in 1883 on the effect of gravity on the cleavage of the frog’s egg, and the conclusions that he drew from his experiments, mark the starting-point for the modern study of experimental embryology.[124] We can trace the influence of Pflüger’s results through most of the more recent work, and one of the conclusions reached by Pflüger, namely, that the material of the egg may be divided by the cleavage planes in any way whatsoever without thereby altering the position of the embryo on the egg, is, I think, one of the most important results that has yet been reached in connection with the experimental work on the egg. Pflüger’s analysis of the factors that direct the development has also an important bearing on the interpretation of the development of a whole embryo from a part of an egg.