No. 383-] THE ADVANCE OF BIOLOGY IN 1896. 869 
this due to the results of Crampton, who showed that in the gas- 
tropod Ilyanassa, when the mesodermal pole cells are removed, 
no mesodermal structures are produced in the embryo. Thus 
there is here, at a certain late cleavage stage, a specialization of 
cells in the germ. The series of specialization, which finds its 
lower extreme in the medusa and its higher extreme in the 
gastropod, is, however, not shown to be one of specialization of 
nuclear material, but rather of cytoplasmic material only. In 
so far as the cytoplasm of the germ is specialized, in so far is 
there preformation in the germ. In so far, on the other hand, 
as the fate of a portion of the germ is determined by its posi- 
tion in the body, development is epigenetic. The variation in 
the degree of preformation is due to the fact that the cytoplasm 
of some eggs exhibits little responsiveness, that of others a 
great deal. New evidence for the unspecialized character of 
the nuclei was given by Wilson, who found that petential 
micromeres of Nereis, forced to become macromeres, had the 
fate of macromeres. Last year it was shown that one-fiftieth 
of an echinoid egg might develop ; this year Lillie showed that 
one-twenty-seventh of a Stentor is capable of forming a new 
individual. Jennings showed that the axis of the spindle in a 
developing rotifer egg is not always placed in the greatest 
extent of the cytoplasm (Hertwig’s law), but occupies various 
positions. The spindle is sometimes stimulated by the form of 
the cell to lie in its long axis, at other times to lie in its short 
axis, and at still other times to lie obliquely. Among attempts 
at a mechanical explanation of ontogenesis were those of Roux, 
who showed that the lines of contact of oil drops resemble those 
of cleavage spheres. The theory of development as a response 
to stimuli was enriched by the experiment of Driesch, which 
showed that the mesenchymatous cells of echinoid gastrulas, 
disarranged by shaking, return to their respective places. Self- 
regulation was illustrated by the observations that variations in 
the embryo are greater than in the adult (whence they must 
gradually become obliterated during development), that with 
shortened range of contraction of a muscle its tendinous part 
is increased at expense of its contractile part, and that a bone 
called upon to carry an abnormally great weight increases in 
