298 NATURAL SCIENCE. April, 



Driesch is strongly opposed to the conception first clearly stated 

 by His, that there exist in the germ organ-forming regions, for 

 the marginal material of a left half-morula may become part of the 

 median region, and eventually part of the right flank of the Pluteus. 

 Moreover, in some cases, the first two segmentation-cells shaken 

 apart give rise to distinct twins, or if the separation be imperfect, to 

 double organisms, at least, to double blastulae. In one case a double 

 Pluteus was obtained from the imperfect separation of the halves of 

 a blastula. 



In a second paper, Driesch gives the results of further experi- 

 ments on the eggs of sea-urchins (Spharechinus granulans and Echinus 

 microtiiheratlaUis). Vejdovsky, in his account of the development of 

 Allolohophora trapezoides — an earthworm whose eggs very frequently 

 form twins — had suggested that the twinning was, perhaps, influenced 

 by warmth, for it was most frequent in warm weather. This sug- 

 gestion (for speculative suggestions are often valuable) prompted 

 Driesch to try the eff"ect of increased warmth on the developing ova 

 of sea-urchins. The result was very striking. Almost all the eggs 

 of Sphavechiniis formed distinct twin blastulae, gastrulae, and even 

 Plutei. In one case a connected twin gastrula was observed. The 

 eggs of Echinus did not respond to the warming ; and sometimes, 

 strange to say, all the warmed eggs of Spharechinus turned out single 

 normal embryos. Still, that there is a relation between warmth and 

 twinning seems likely. ■ 



Many experiments were made with the four-celled stage. If one 

 cell was shaken off, or pricked to death, the three-quarters left 

 almost always developed quite normally into Plutei. Yet the details 

 of segmentation were of course different. To remove one of the first 

 four cells does not hinder development to any appreciable extent. 



But, more than that, an isolated cell of the four-celled stage, 

 isolated by bursting the other three, develops as if it were still in its 

 natural alliance. Very few of these, however, got beyond the 

 blastula-stage, but two became Plutei. A quarter, a half, or three- 

 quarters of the four-celled stage may therefore form a fully developed 

 larva. 



Selenka, in his studies on the development of marine Planarians, 

 had observed that increased temperature produced deviations from 

 the normal segmentation. This led Driesch to experiment. He 

 found that with increased temperature the formation of smaller cells 

 or micromeres is wholly or partially inhibited, and that in other ways 

 the segmentation may be disturbed, but the striking fact is that 

 slight changes in the position of the segmentation-cells, and even a 

 modified type of segmentation, do not hinder the development of 

 a normal organism. 



Driesch proceeded to experiment on the effect of pressure, which 

 was supplied by the weight of a cover-glass resting in part upon the 

 ova. The nuclear spindles were disposed at right angles to the direction 



