In this chapter we shall attempt the task among others of showing how, on the basis of the simple physico­chemical structure of the unfertilized egg, the main organ of self-preserva­tion of the organism, the intestine, is formed through the mere process of cell division and growth. Cell division is the most general of the specific func­tions of living matter and it is the basis underlying the differentia­tion of the comparatively simple structure of the egg into a more complex organism. If cell division and growth were equal in all parts of the egg no differentia­tion would be possible, but the different regions of the unfertilized egg contain different constituents and these, probably on account of their chemical difference, do not all begin to grow or divide simultaneously and equally.

Fig. 9

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.

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.

Fig. 10   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.

Fig. 12

The cell division proceeds and the cells become smaller and smaller and all gather at the surface of the egg, thus forming a hollow sphere. It is not known what brings about this gathering of the cells at the surface, whether it is protoplasmic creeping or streaming or whether the cells are held by a jelly-like layer which covers the surface of the egg (hyaline membrane) (Fig. 13). Then the cilia are formed at the external surface of these cells and the egg begins to swim; we say it has reached the first larval, the so-called blastula stage. This happens according to Driesch after the tenth series of cell divisions, when the number of cells is theoretically 1024, in reality not quite so many (between 800 and 900). The next step consists in the cells derived from the material A (mesenchyme and micromeres) gliding into the hollow sphere, where they form a ring, the physico­chemical process responsible for this gliding being yet unknown. At the opening of this ring an active growing of the cells of the entoderm into the hollow sphere takes place and the hollow cylinder formed by this growth is the intestine (Fig. 14). Why the cells grow into the hollow sphere and not into the opposite direc­tion is unknown. The next step is the forma­tion of a skeleton by the forma­tion of crystals consisting of the CaCO3 by the mesenchyme cells surrounding the intestine. For the establishment of the principle in which we are interested the descrip­tion of morphogenesis need not be carried farther.