Regeneration . s found that a double anterior end is produced. As theembryo develops it tends to elongate, and in consequence the mate-rial is pushed forward on each side of the ligature. A double head isthe result. The extent of the doubhng depends on the depth of theconstriction between the halves. In the most extreme cases two com-plete heads are formed with an inner nasal pit, eye, and ear on eachhead, as well as the normal outer ones. The results show that evensuch complicated structures as the eyes and ears, etc., may arise 228 REGENERA TION from parts of the body where they never appear
Regeneration . s found that a double anterior end is produced. As theembryo develops it tends to elongate, and in consequence the mate-rial is pushed forward on each side of the ligature. A double head isthe result. The extent of the doubhng depends on the depth of theconstriction between the halves. In the most extreme cases two com-plete heads are formed with an inner nasal pit, eye, and ear on eachhead, as well as the normal outer ones. The results show that evensuch complicated structures as the eyes and ears, etc., may arise 228 REGENERA TION from parts of the body where they never appear under normalconditions. A series of experiments that have been made on the eggs of sea-urchins has led to equally important results. The earliest experi-ments are those of O. and R. Hertwig, who, in addition to studyingthe effect of different drugs on the developing ^g^, found thatfragments of the eggs of sea-urchins, obtained by violently shak-ing the eggs in a small vial, could give rise, if they contained a. Fig. 64. — Sea-urchin egg and embryo. A. Two-cell stage. B. Same, with blastoraeres Two half-sixteen-cell stages. C. Open half-blastula stages. D. One of last, later stage,closed blastula of half size. H. Gastrula of half size. F. Whole pluteus of half size. H. Ahall-sixteen cell dividing in same way as a whole egg (eight cell). /. Whole egg at sixteen-cell stage. nucleus, to small whole embryos. Boveri made the important discov-ery in 1889 that if a non-nucleated piece of the of the sea-urchinis entered by a single spermatozoon, the piece develops into a wholeembryo of a size corresponding to that of the piece. Fiedler, in 1891,separated the first two blastomeres by means of a knife, and foundthat the isolated blastomere divides as a half, but he did not succeedin obtaining embryos from the halves. Driesch has made many ex-periments, beginning in 1891, with the eggs and embryos of the sea-urchin. He separated the first two blastomeres (91) by means
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