[129] Crampton, H. E., _New York Academy of Sciences_, 1894; Kofoid, C. A., _Proc. Am. Acad. Arts and Sciences_, 1894, xxix. [130] Conklin, E. G., _Anat. Anzeig._, 1903, xxiii., 577; _Heredity and Environment in the Development of Man_. Princeton, 1915, p. 171. E. B. Wilson[131] has found a marked differentiation in the eggs of some annelids and molluscs. He isolated the first two blastomeres of the egg of _Lanice_, an Annelid. These two blastomeres are somewhat different in size; from the larger one of the first two blastomeres, the segmented trunk of the worm originates. Wilson found that when either cell of the two-cell stage is destroyed, the remaining cell segments as if it still formed a part of an entire embryo.[132] The later development of the two cells differs in an essential respect, and in accordance with what we should expect from a study of the normal development. The posterior cell develops into a segmented larva with a prototroch, an asymmetrical pre-trochal or head region, and a nearly typical metameric seta-bearing trunk region, the active movements of which show that the muscles are normally developed. The pre-trochal or head region bears an apical organ, but is more or less asymmetrical, and, in every case observed, but a single eye was present, whereas the normal larva has two symmetrically placed eyes. The development of the anterior cell contrasts sharply with that of the posterior. This embryo likewise produces a prototroch and a pre-trochal region, with an apical organ, but produces no post-trochal region, develops no trunk or setæ, and does not become metameric. Except for the presence of an apical organ, these anterior embryos are similar in their general features to the corresponding ones obtained in _Dentalium_. None of the individuals observed developed a definite eye, though one of them bore a somewhat vague pigment spot.