Regeneration

by Thomas Hunt Morgan

In those cases in which the ligature lies in the median plane of the embryo, it is found that a double anterior end is produced. As the embryo develops it tends to elongate, and in consequence the material is pushed forward on each side of the ligature. A double head is the result. The extent of the doubling depends on the depth of the constriction between the halves. In the most extreme cases two complete heads are formed with an inner nasal pit, eye, and ear on each head, as well as the normal outer ones. The results show that even such complicated structures as the eyes and ears, etc., may arise from parts of the body where they never appear under normal conditions.

Fig. 64.—Sea-urchin egg and embryo. A. Two-cell stage. B. Same, with blastomeres separated. G. Two half-sixteen-cell stages. C. Open half-blastula stages. D. One of last, later stage, closed blastula of half size. E. Gastrula of half size. F. Whole pluteus of half size. H. A half-sixteen cell dividing in same way as a whole egg (eight cell). I. Whole egg at sixteen-cell stage.

A series of experiments that have been made on the eggs of sea-urchins has led to equally important results. The earliest experiments are those of O. and R. Hertwig, who, in addition to studying the effect of different drugs on the developing egg, found that fragments of the eggs of sea-urchins, obtained by violently shaking the eggs in a small vial, could give rise, if they contained a nucleus, to small whole embryos. Boveri made the important discovery in 1889 that if a non-nucleated piece of the egg of the sea-urchin is entered by a single spermatozoon, the piece develops into a whole embryo of a size corresponding to that of the piece. Fiedler, in 1891, separated the first two blastomeres by means of a knife, and found that the isolated blastomere divides as a half, but he did not succeed in obtaining embryos from the halves. Driesch has made many experiments, beginning in 1891, with the eggs and embryos of the sea-urchin. He separated the first two blastomeres (’91) by means of Hertwig’s method of shaking the eggs, and studied the development of the isolated blastomeres. He found that the cleavage was strictly that of a half, and not like that of a whole egg. The normal egg divides into two, four, and eight equal parts. At the next division, four of the cells divide very unequally, producing four very small cells, the micromeres, at one pole. The four cells of the other hemisphere divide equally ([Fig. 64], I). The isolated blastomere divides at first into two equal parts, then again into equal parts. At the next division two of the cells produce micromeres and two divide equally ([Fig. 64], G). This is exactly what happens at this division in each half, if the blastomeres are not separated. In later stages a half-sphere is formed that is equivalent to half of the normal sphere ([Fig. 64], C). The open side corresponds to the side at which the half would have been united to the other half. Thus up to this point a half-cleavage and a half-blastula have appeared.[114]

In later stages the open half-blastulæ close in, producing a whole sphere that becomes perfectly symmetrical ([Fig. 64], D). A symmetrical gastrula develops ([Fig. 64], E) by the invagination of a tube at one pole, and a symmetrical embryo is formed ([Fig. 64], F) that resembles the normal embryo except in point of size.

Driesch has also found that a number of twin embryos arise from the shaken eggs. They arise from eggs whose blastomeres have been disturbed or shifted, so that each produces a small whole embryo, the two embryos being united to each other in various ways.

In a second paper, published in the following year, Driesch extended his experiments, and attempted to discover how far the “independence” of the blastomeres extends; i.e. he tried to find out if all the blastomeres resulting from the cleavage are alike. When one of the first four cells is separated from its fellows by shaking, it continues to divide, in most cases as a quarter, and produces later a small spherical blastula. Many of these blastulæ, although apparently healthy, never develop further, although they may remain alive for several days. In one experiment only eight out of twenty-six reached the pluteus stage, with a typical digestive tract and skeleton.

From these experiments Driesch drew the important conclusion that the cleavage cells or blastomeres of the sea-urchin’s egg are equivalent, in the sense that if they were interchanged a normal embryo would still result. A somewhat similar view is expressed in the dictum that the position of a blastomere in its relation to the others determines what part it will produce, if its position is changed it gives rise to another part, etc.,—or, expressed more concisely, the prospective value of a blastomere is a function of its position.[115] Driesch extended these experiments further in 1893. His aim was to separate different groups of cells at the sixteen-cell stage in order to see whether the cells around the micromere pole (or “animal pole”) if separated from those of the opposite (or “vegetative pole”) could produce a whole embryo, etc. Eggs whose membranes had been removed by shaking immediately after fertilization were allowed to develop normally to the sixteen-cell stage and were then shaken into pieces. Amongst the groups of cells that were present those that contained the micromeres were picked out. It was found that they give rise to whole embryos. In order to obtain cells that belong to the vegetative hemisphere, the blastomeres were shaken apart at the eight-cell stage, and those groups of cells that in later divisions did not produce micromeres were isolated. From these also whole embryos develop. The results show that the cells of both hemispheres are able to produce whole embryos, and that at the sixteen-cell stage the different parts of the egg are still capable of producing all parts of the embryo. It is important to observe that the results of the experiment do not show that if the normal development goes on undisturbed any part of the egg may become any part of the embryo, for it is highly probable that a definite region of the egg may always produce a definite part of the embryo. The results do show, however, that, even if this is true, any cell has the power of producing any or all parts of the embryo if the normal conditions are changed.

In connection with these experiments Driesch discussed the factors that determine the axial relations of the embryo. If all the cells have the power of producing all parts, what determines in the normal development, and also in the development of a part of the whole, the axial relations of the embryo? Driesch assumed that the egg has a polar structure, and that the same polarity is found in all parts of the protoplasm. Around this primary axis all the parts are alike or isotropous.[116] The origin of the mesenchyme and the position of the archenteron, that develop at one pole, are determined by the polarity of the protoplasm. The plane of bilateral symmetry may appear in any one of all the possible radial planes around the primary axis. The selection of a particular one is due to some accidental difference in the structure of the protoplasm, or to some external factor. In later papers Driesch modified this view, and assumed that along with the primary polarity a bilateral structure also exists in the protoplasm.