The part of the work of Driesch and Morgan, that has a special bearing on the interpretation of the one-half and one-fourth development of the isolated blastomeres, is that in which some experiments are described which consisted in cutting off portions of the unsegmented egg. If a fertilized but unsegmented egg is cut in two by means of a small pair of scissors, the part that contains the nucleus may segment, and give rise to an embryo. The division is generally like that of a part, and in such cases an incomplete embryo develops. The embryo may have fewer rows of swim-plates than has the normal embryo, and fewer endodermal sacs, and the stomach may be in an eccentric position. The embryos resemble in every respect the incomplete embryos from isolated blastomeres. It is important to note that although the embryos from isolated blastomeres resemble those from pieces of the segmented egg, in the former case the nucleus has divided once, and each blastomere contains half of the original nucleus, while in the latter case the entire segmentation nucleus is present in the piece. These facts seem to show that in this egg the incomplete development is directly connected with the protoplasm, and not with the nucleus,—a view that is maintained by Driesch and Morgan in connection with these experiments.

It was found in one or two instances that the nucleated pieces divided in the same way that the whole egg did, except that the blastomeres are proportionately smaller. From pieces of this kind whole embryos of small size developed. In this case we must suppose that the protoplasm has succeeded in rearranging itself into a new whole of smaller proportions.[123]

Crampton (’96) has shown in a mollusk, Ilyanassa obsoleta, that when a blastomere is separated from the rest, the cleavage proceeds as though the blastomere or its products were still present, and the larva is defective in those organs that are normally derived from that blastomere. These results are in line with those on the ctenophore egg. Fischel (1900) has also made some experiments on the segmented egg of the ctenophore, and has confirmed several of the results obtained by Driesch and Morgan. In addition he has tried the effect of disturbing the first-formed cells by pushing them over each other, so that their relative positions are changed. He finds as a result that the paddles, sense organ, etc., appear in unusual positions, and the latter may be doubled. This shows that we must regard the material or structural basis of the organs as present very early in the different parts of the egg, and that the organs develop without much regard to their relation to other organs.

Ziegler (’98) has also made some observations on the egg of this same ctenophore, that bear directly on some of the questions here raised. His study of the cleavage shows that the micromeres arise from the part of the egg that is opposite the pole at which the first cleavage furrow appears—the animal pole. Fischel’s results have shown that the paddles and the sense organs arise from these micromeres, for, if the latter are displaced the former are also.

Ziegler performed the experiment of cutting off that part of an egg (which has just begun to divide) lying opposite the region in which the first furrow has appeared. In this way there was removed from the unsegmented egg the part from which the micromeres develop. Ziegler found that the micromeres still arise, and that from such pieces larvæ develop that have eight rows of paddles and four endodermal sacs. In one case two of the sacs were smaller than the others; in another case one of the four was very much smaller than the rest. In another operation a large piece was cut from the egg, leaving a small nucleated piece that divided into two blastomeres of unequal size. An embryo developed from this small piece with four endodermal sacs, and only four well-developed rows of paddles. The four rows of paddles that were lacking were represented by two groups of a few plates each.

Ziegler gives a different interpretation of these results from that which Driesch and Morgan have offered. He interprets the last experiment, in which after the operation the piece divided into two unequal parts, and only four rows of paddles appeared, as meaning that the development of these organs on the smaller part is suppressed on account of the small size of the part. If the part had been still smaller all trace of the missing paddles might disappear, as he thinks was the case in certain experiments of Driesch and Morgan. There can be, I think, little doubt that if a piece is small enough, the result would follow as Ziegler supposes. It does not seem probable, however, that the pieces were really below the lower limit in the experiments of Driesch and Morgan, since the smaller blastomere was in one case as large as the whole piece (i.e. as both blastomeres taken together) in one of Ziegler’s experiments.

Ziegler’s results show very clearly that we are not obliged to think of the substance of the micromeres as laid down in the protoplasm of the egg, and hence there is no ground for supposing the substance of the paddles is necessarily present in the vegetative hemisphere of the egg. His results show that if the vegetative part is cut off, micromeres and paddles are still formed, although that part of the egg substance from which they normally arise has been removed. It should be pointed out, in this connection, that Driesch and Morgan did not suppose that the bases of the micromeres, or of the paddles, are actually laid down in a definite part of the protoplasm of the egg. They had also observed that in some cases whole embryos arose after a part of the egg had been removed, and this they attributed to the symmetrical position of the cut in relation to the organization of the egg. Ziegler’s operations were made more or less in this symmetrical plane, excepting the one that gave rise to an incomplete embryo. Driesch and Morgan held that the formative factors become localized in the protoplasm, rather than arise from the nucleus, but pointed out that these observations do not lead to His’s conclusion of localized germ areas in the egg.

CHAPTER XII
THEORIES OF DEVELOPMENT

The experimental work that Pflüger carried out in 1883 on the effect of gravity on the cleavage of the frog’s egg, and the conclusions that he drew from his experiments, mark the starting-point for the modern study of experimental embryology.[124] We can trace the influence of Pflüger’s results through most of the more recent work, and one of the conclusions reached by Pflüger, namely, that the material of the egg may be divided by the cleavage planes in any way whatsoever without thereby altering the position of the embryo on the egg, is, I think, one of the most important results that has yet been reached in connection with the experimental work on the egg. Pflüger’s analysis of the factors that direct the development has also an important bearing on the interpretation of the development of a whole embryo from a part of an egg.

Pflüger found that in whatever position the frog’s egg is turned before it begins to divide, the first two planes come in vertically, and the third horizontally, and that later the smallest cells are always formed in the upper hemisphere. He concluded, therefore, that gravity has some sort of influence in determining the position of the planes of cleavage. Pflüger observed that the position of the median plane of the body of embryos that have developed from eggs turned into unusual positions does not, as a rule, correspond to the plane of the first cleavage, but that the embryo generally lies on that meridian of the egg that passes through the primary egg axis and the highest point of the egg in its new position. Since any meridian may happen to be placed uppermost, the embryo may, therefore, develop upon any one of the primary meridians, and hence the material must be isotropous around the primary axis. Furthermore, since the embryo appears always below the middle of the egg, in whatever position the egg may lie, we must conclude that in each meridian the material is also isotropic.