502 The American Naturalist. [May, 
The action of weak salt solution is thus apparent as a partial inhib- 
ition of the normal developmental processes. 
A considerable part of the paper is taken up with a consideration of 
the differences of view between Weismann and Hertwig, and the ap- 
plication of these new facts to the epigenetic conception of develop- 
ment. 
Stimuli in Embryology.—Curt Herbst’ reviews all the various 
forms of movements that are called forth in the lower animals and in 
plants by the action of heat, light, chemical bodies, etc., and known com- 
monly as thermotaxis, phototaxis, chemotaxis, etc., and then advances 
a plea for regarding such responses to stimuli as important factors in 
the development of the individual. 
Physiological stimuli are thus to be regarded as important factors in 
the processes of animal ontogeny. Just as a plant or animal cell may 
move to or away from the source of light, heat or chemical action and 
just as a plant may bend toward or away from such agents or respond 
to gravity or to moisture, so, Herbst thinks, may cells and organs in 
the embryo move or change form in response to various stimuli. 
He would thus explain many well known facts; the migration of 
nuclei to the surface of an insect egg may be the result of positive 
ærotaxis, that is, the response of the nuclei to stimuli coming from the 
more abundant oxygen near the surface of the egg. The movements 
of vitellophags likewise may be the results of definite stimuli. 
n later stages the remarkable collecting of mesenchyma cells to in- 
vest nerve processes, etc., that is, the formation of the sheath of Swan 
and the neurilemma as well as the coats of blood vessels may again be 
due to migrations under the directive influence of stimuli. Even the 
outgrowth of nerve fibres to the end organs (generally regarded as 
actually taking place) may not be along the lines of least resistance but 
controlled by directive stimuli. 
All this, it will be observed, is an outgrowth of the observations 
upon lithium salts and echinoderm larve noticed in this journal for 
December, 
ê Biologische Centralblatt, Nov., 1894. 
