228 



KEGENERA TION 



from parts of the body where they never appear under normal 

 conditions. 



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 studying 

 the effect of different drugs on the developing egg, found that 

 fragments 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 blastomeres separated. 

 G. Two half-sixteen-cell stages. C. Open half-blastula stages. D. One of last, later stage, 

 closed blastula of half size. . Gastrula of half size. F. Whole pluteus of half size. H. A 

 hall-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 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 ex- 

 periments, beginning in 1891, with the eggs and embryos of the sea- 

 urchin. He separated the first two blastomeres ('91) by means of 



