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 blastula. 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 (Spherechinus granularis and Echinus 
microtuberculatus). Vejdovsky, in his account of the development of 
Allolobophova trvapezoides—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 effect of increased warmth on the developing ova 
of sea-urchins. The result was very striking. Almost all the eggs 
of Spherechinus formed distinct twin blastule, gastrule, 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 Spherechinus 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 
