Boveri[117] found that in the unfertilized egg of the sea urchin _Strongylocentrotus lividus_ at Naples a definite structure is indicated by the fact that the yellowish-red pigment is not equally distributed over the whole surface of the egg but is arranged in a wide ring from the equator almost to one of the poles. Thus three zones can be recognized in the egg (Fig. 9), a small clear cap _A_ at one pole, a pigmented ring _B_, and the rest again unpigmented _C_. Observation has shown that each one of these regions gives rise to a definite constituent of the egg: _A_ furnishes the mesenchyme from which the skeleton and the connective tissue originate; _B_ is the material for the formation of the intestine, and _C_ gives rise to the ectoderm. [117] Boveri, Th., _Verhandl. d. physik.-med. Gesellsch._, Würzburg, 1901, xxxiv., 145. [Illustration: FIG. 9] The pigment is only at the surface of the egg, and its collection at _B_ indicates only that the material in _B_ differs physicochemically from _A_ and _C_. The real determiners of the three different groups of organs are three different groups of substances whose distribution is approximately but probably not wholly identical with the regions indicated by distribution of pigment. The intestine-forming material is probably not entirely lacking in _C_ but is contained here in a lower concentration and probably the more so the greater the distance from _B_; and the same may probably be said for the substances determining mesenchyme and ectoderm formation. Hence the unfertilized egg contains already a rough preformation of the embryo inasmuch as the main axis of the embryo and the arrangement of its first organs are determined. [Illustration: FIG. 10] [Illustration: FIG. 11] After the egg is fertilized the cell divisions begin. The first division is as a rule at right angles to the stratification of the egg, each of the two cells contains one-half of the pigment ring (and of each of _A_ and _C_) (Fig. 10), and after the next division each contains one-fourth of the pigmented part. Each of the four cells is a diminutive whole egg since each contains the three layers in the normal arrangement (Fig. 11). The next divisions bring about an unequal division of the material. Four cells will be formed of ectoderm material _C_ and only little intestine material _B_, the other four cells containing _B_ and _A_. These latter form at the next division four very small colourless cells, the so-called micromeres, _A_ (Fig. 12), from which the mesenchyme, skeleton, and connective tissue are formed, four larger cells, _B_, from which the intestine is formed, and eight cells, _C_, from which the ectoderm will arise. The separation of the three groups of substances is probably not as complete as our purely diagrammatic drawing (Fig. 12) indicates. [Illustration: FIG. 12]