Pflüger’s experiments were made by changing the conditions under which the egg develops in order to determine what factors control the development. Since these experiments were made with whole eggs, the problems of regeneration were not directly involved in his results, although his conclusions are of great importance in connection with questions concerning the regeneration of the egg. A part of Roux’s work dealt directly with the development of a new organism from a piece of the egg or of the embryo. Roux’s principal discovery[107] (’88) was that a half-embryo develops from either of the first two blastomeres of the frog’s egg, if the other blastomere has been injured or destroyed, but that subsequently the missing half of the embryo is “post-generated.” Roux was led to this experiment by his discovery that the plane of the first cleavage of the egg corresponds very often to the median plane of the body of the embryo.[108] This relation suggested that there might be some causal connection between the two phenomena in the sense that the first cleavage plane divides the material for the right side of the body from that of the left side. In a descriptive sense this would be, of course, true if the two planes do really correspond, and if there was no later shifting of material across the middle line, but whether the two phenomena are causally connected, or are merely due to a coincidence, could only be determined by further experiment. The observations themselves are not beyond question, for the two planes do not always coincide, and may be even ninety degrees apart. These cases of divergence were thought by Roux to be due to an unobserved shifting of the developing embryo, but it is improbable that all cases can be accounted for in this way. [Illustration: FIG. 61.--After Roux. _A._ Section of semi-blastula of frog’s egg. _B._ Half-embryo. _C._ Cross-section of last (reversed right and left in _B_ and _C_). _D._ Anterior half-embryo.]