It is significant in this connection to find that Hertwig obtained other embryos that show a condition of “spina bifida.” In these there is an exposure of yolk in the mid-dorsal line between the halves of the medullary folds. Still other embryos in the same series of experiments were only slightly injured, and developed nearly normally. In these cases, Hertwig thinks, the blastomere that was stuck had been only slightly injured, and had partly developed. I have also often observed in this experiment that the injured blastomere may segment and add cells to the developing half, but in such cases the development of the injured half may be less regular than is that of the uninjured half. It seems to me not improbable that in several of the embryos described by Hertwig both blastomeres have taken part in the development. The main points of difference between the results of Roux and of Hertwig cannot, however, be explained in this way, and the explanation is to be found in another direction. Hertwig emphasizes the view that the injured blastomere is not dead, but exerts an influence upon the other half--an influence of the same kind as that which the yolk of a meroblastic egg has on the protoplasmic portion of the egg from which the embryo arises. He ventured to prophesy that if the injured yolk mass could be entirely removed, the uninjured blastomere would produce a normal embryo without defect, and one like the normal embryo in every respect except in size.[111] [Illustration: FIG. 63.--_A._ After Wetzel. Section through an egg (blastula stage) reversed at two-celled stage. _B._ After Schultze. Double embryo, from reversed two-celled stage, united ventrally. _C, C¹._ Two views of another double embryo (united dorsally). _C²._ Cross-section through last. _D._ After Wetzel. Double embryo united laterally. _D¹._ Section through same.]