Fig. 13    Fig. 14

This principle which is under discussion here is the development of a purposeful arrangement of organs out of the egg. If we assume that the egg consists of homogeneous material we are indeed confronted with a riddle. Since the facts contradict such an assump­tion but show, as Boveri has pointed out, a prearrangement which allows us to indicate in the unfertilized egg already the exact spot where the intestine will grow into the blastula cavity, we are on solid physico­chemical ground, although many ques­tions of detail cannot yet be answered. Such a preforma­tion as Boveri has demonstrated is only conceivable if the material of the egg has not too high a degree of fluidity; we may consider it as consisting essentially of a semi-solid gel which is not homogeneous throughout the egg but divided into three strata.

2. Lyon[118] tried to ascertain whether by centrifuging the sea-urchin egg it was possible to modify its structure and thereby affect the later embryo. He and subsequent experi­menters found that it only is possible to change the posi­tion of the nucleus and the distribu­tion of the pigment in the egg. It follows from this that the nucleus and the pigment are suspended in rather fluid material, the former in the centre, the pigment at or near the surface. The posi­tion of the nucleus determines the first plane of segmenta­tion, since the nuclear division precedes the division of the cytoplasm of the egg and the plane of nuclear division becomes also the plane of the division of the whole egg—a point which need not be discussed here. It was found, however, by Lyon and the subsequent investigators that the place where the micromeres are formed and where the intestine of the embryo later originates is little influenced by the centrifuging of the egg. The localiza­tion of this spot must therefore be determined by a structure sufficiently solid not to be shifted by the centrifugal force. The intestinal stratum in the egg contains the forerunners of the tissues which secrete hydrolyzing enzymes, e. g., trypsin into the digestive tract.

When the surrounding solu­tion is altered in constitu­tion or when the temperature is too high, the intestine instead of growing into the hollow sphere grows outside, we get an evagina­tion instead of an invagina­tion of the intestine. Such larvæ may live for a few days but they cannot grow into a living organism. The forces which make the intestine grow into the hollow sphere are unknown; it may possibly be only the difference between the tension on the external and internal surfaces of the hollow sphere; under normal condi­tions, the resistance on the inner surface being smaller, the intestine grows into the hollow sphere.

The intestine is one of the organs required for the self-preserva­tion of a more complicated organism, in fact a higher organism without a digestive tract is not capable of living for any length of time. In the gastrula—i. e., the blastula with an intestine—we have an organism which is durable, but the processes leading up to the forma­tion of the intestine are so simple that it is difficult to understand why the assump­tion of a “supergene” should be required in this case.

Fig. 15

3. Driesch[119] was the first to show that if we isolate one of the first two cells of a dividing egg each develops into a whole embryo of half size. This is perfectly intelligible, since each of the two cells contains all the three layers in the normal arrangement (Fig. [10]). The cells divide and the cells having the tendency to creep to the surface of the mass arrange themselves in a hollow sphere, the blastula. Since micromeres and intestine material are present and in their normal posi­tion an intestine will grow into the blastula and a whole organism will result. All of this is as necessary as is the forma­tion of one embryo from the whole egg material. Yet the two half-embryos betray their origin from two cleavage cells of the same egg, in that the two gastrulæ formed are often if not always symmetrical to each other (Fig. 15), as the writer had a chance to observe in the egg of Strongylo­centrotus purpuratus[120] in the following experi­ment. The eggs of the sea urchin Strongylo­centrotus purpuratus are put soon after fertiliza­tion into solu­tions which differ from sea water in two points; namely that they are neutral or very faintly acid (through the CO2 absorbed from the air) instead of being faintly alkaline, and second, that one of the following three constituents of the sea water is lacking; namely: K, Na, or Ca. When the eggs are allowed to segment in such a solu­tion the first two cleavage cells are as a rule in a large percentage of cases—often as many as ninety per cent.—separated from each other, and when the eggs are put into normal sea water (about twenty minutes after the cell division) each cell develops into a normal embryo. In a number of cases the embryos remained inside the egg membrane and did not move until after the invagina­tion of the intestine was far advanced; in such cases it was found quite often that the invagina­tion began at the plane of cleavage at symmetrical points of the two embryos, and the growth of the intestine was symmetrical in both embryos.

This symmetry is probably due to the following fact: the first cleavage plane goes through that spot where the intestine grows into the blastula cavity. If the micromere material does not change its posi­tion after the two cleavage cells are separated and the new blastulæ do not become completely spherical the symmetry which we observed is bound to occur. The occurrence is a confirma­tion of Boveri’s observa­tion. It is natural that Driesch also found that each cell in the four-cell stage should give rise to a full embryo, since each of these cells is in reality a diminutive egg containing the three strata in the right arrangement. When, however, the cells of the eight- or sixteen-cell stage were isolated Driesch’s results were different. In this case the isolated cells from the ectoderm material did no longer all form a gastrula; when such a cell still formed a gastrula it was probably due to the fact that it contained some entoderm material; while the cells taken from the entoderm region all formed embryos and therefore contained ectoderm material.[121] The isolated ectoderm cells of a blastula could no longer form an intestine; they were lacking the entoderm material. It looks as if a gradual migra­tion of all the entoderm material from the ectoderm into the entoderm took place during the blastula forma­tion.