The dynamics of living matter . dynamicsofliving1906loeb2 Year: 1906 HEREDITY 193 much as they show how twins, triplets, and quadruplets can originate from one egg. The lack of any complicated structure in the unseg- mented egg is, I believe, evidenced very strikingly in the following observation. I have mentioned in a former lecture that if the egg of a sea urchin (Arbacia) is put into diluted sea water (equal parts of sea Water and distilled water), many eggs will burst, and part of the protoplasm will flow out, without necessarily being separated from the rest of the egg. In this case t
The dynamics of living matter . dynamicsofliving1906loeb2 Year: 1906 HEREDITY 193 much as they show how twins, triplets, and quadruplets can originate from one egg. The lack of any complicated structure in the unseg- mented egg is, I believe, evidenced very strikingly in the following observation. I have mentioned in a former lecture that if the egg of a sea urchin (Arbacia) is put into diluted sea water (equal parts of sea Water and distilled water), many eggs will burst, and part of the protoplasm will flow out, without necessarily being separated from the rest of the egg. In this case the normally spherical egg is transformed into a double sphere or a dumb-bell-shaped mass (Figs. 35 and 36). This mass may give rise to a single embryo, or to 'Siamese twins,' and whether the one or the other occurs depends upon the width of the piece ab (see Figs. 35 and 36) that connects the two spheres.* If this piece is very narrow, as in Fig. 36, twin blastulae will originate from such an egg; if it is wide, as in Fig. 35, only a single embryo will develop from it. Why this should be so can be readily recognized. We have already stated that the cells have a tendency to creep to the periphery of the egg, thus leaving an empty space in the center which becomes the blastula cavity. When the connecting piece is very narrow, it will be filled with cells, and the two segmentation cavities can and will remain separate, and two blastula? will be formed (Fig. 37). If, however, the piece ab is wide (Fig. 38), an open space will be left in this connecting piece, by which the two blastula cavities communicate, and in this case only one blastula cavity, and hence only one embryo will be formed. The distorted dumb-bell-shaped blastula soon becomes spherical (through the secretion under pressure of liquid into the interior), and a normal larva results. These facts prove that as far as the formation of the blastula is concerned there is no preformed structure of any high degree of complica
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