EXPERIMENTAL EMBRYOLOGY 331 



was brought about by a qualitative division of the germ-plasm 

 contained in the nucleus, and that the complicated process of 

 karyokinetic or mitotic division of the nucleus was essentially 

 adapted to this end. He conceived that development 

 proceeded by a mosaic-like distribution of potencies to the 

 segmentation-cells, that, for instance, the first segmentation 

 furrow separated off the material and potencies for the right 

 half of the embryo from those for the left half. He had tried 

 to show experimentally that the first furrow in the frog's egg 

 coincided with the sagittal plane of the embryo, 1 and his later 

 success in obtaining a half-embryo from one of the first 

 two blastomeres seemed to establish the " mosaic theory " 

 conclusively. 



Roux's needle-experiment aroused much interest, especially 

 as Weismann's theory of heredity was then being keenly 

 discussed. Chabry had published in 1887 some interesting 

 results on the Ascidian egg, 2 which strongly supported the 

 Roux-Weismann theory. Considerable astonishment was 

 therefore caused by Driesch's announcement in 1891 3 that 

 he had obtained complete larvae from single blastomeres of 

 the sea-urchin's egg isolated at the two-celled stage. He 

 followed this up in the next year 3 by showing that whole 

 embryos could be produced from one or more blastomeres 

 isolated at the four-cell stage. Similar or even more striking 

 results were obtained by E. B. Wilson on Amphioxusf and 

 Zoja on medusae. 5 Driesch succeeded also in disturbing the 

 normal course and order of segmentation by compressing the 

 eggs of the sea-urchin between glass plates, and yet obtained 

 normal embryos. Similar pressure-experiments were carried 

 out on the frog by O. Hertwig, 6 and on Nereis by E. B. 

 Wilson, 7 with analogous results. 



In 1895 O. Schultze 8 showed that if the frog's egg is held 

 between two plates and inverted at the two-celled stage 



1 Bresl. iirtz. Zeitschr., 1885. 



2 Journ, de VAnat. et de la Physiologic, xxiii., 1887. 



3 Zeits.f. wzss. ZooL, liii., 1891 and 1892. 



4 Journ. Morph., viii., 1893. 



5 Arch. f. Ent.-Mech., i., 1895 5 ii-> l8 9 6 - 



6 Arch. f. mikr. Ana/., xliii., 1893. 



7 Arch.f. Ent.-Mech., iii., 1896. 

 s Arch.f. Ent.-Mech., i., 1895. 



