This result shows that from the beginning of development the material for the trunk region is mainly localized in the posterior cell; and, furthermore, that this material is essential for the development of the metameric structure. The development of this animal is, therefore, to this extent, at least, a mosaic work from the first cleavage onward--a result that is exactly parallel to that which I earlier reached in _Dentalium_, where I was able to show that the posterior cell contains the material for the mesoblast, the foot, and the shell; while the anterior cell lacks this material. I did not succeed in determining whether, as in _Dentalium_, this early localization in _Lanice_ pre-exists in the unsegmented egg. The fact that the larva from the posterior cell develops but a single eye, suggests the possibility that each of the first two cells may be already specified for the formation of one eye; but this interpretation remains doubtful from the fact that the larva from the anterior cell did not, in the five or six cases observed, produce any eye. [131] Wilson, E. B., _Science_, 1904, xx., 748; _Jour. Exper. Zoöl._, 1904, i., 1, 197. [132] The reader will notice the absence of “regulation.” Conklin has established the existence of a definite structure in the unfertilized eggs of Ascidians, Amphioxus, and many molluscs. In all cases the results of the isolation of the first blastomeres seem to agree with the demonstrable structure of the unfertilized egg. 5. These examples may suffice to show that the egg has from the beginning a simple structure, and we will now point out by which means further differentiation may come about. Sachs suggested that all differentiation and the formation of every organ presupposes the previous existence of specific substances responsible for the formation. These substances which are now called internal secretions or hormones develop gradually during embryonic development. What exists first is a jelly-like block of protoplasmic material with a varying degree of viscosity and with just enough differentiation to indicate head and tail end, a right and left, and a dorsal and ventral side of the future embryo. Aside from such simple differences phenomena of protoplasmic streaming contribute to the further differentiation. Such streaming begins, according to Conklin,[133] in the egg just before fertilization when the surface layer of the egg protoplasm streams to the point of entrance of the sperm, and these movements may lead to the segregation of different kinds of plasma in different parts of the egg and to the unequal distribution of these substances in different regions of the egg.