292 The American Naturalist. [Mareh, 
Most interesting differences obtain amongst the various salts of 
lithium in the strength of their action. If the eggs of the same sea 
urchin are treated simultaneously with the various salts we find that 
at a given time the larve were not all equally far advanced, equal 
amounts of the some salts acting sooner than others in producing the 
lithium larva, and larger amounts of some salts being necessary to 
produce the same results as smaller amounts of other salts of lithium. 
From a table of such experiment the author concludes that Li Cl, 
Li NO,, Li Br, and Lil are less and less active in this order which is 
also the order of increasing molecular weights. Thus in these experi- 
ments where the same per centage of salts was always used the heavy 
molecules were less numerous and less active ; the action of these salts 
in producing the lithium larva diminishes with the number of mole- 
cules used. This rule, however, finds an exception in Li, SO,. 
In NaCl, Na NO,, Na Br, and NaI as well asin K Cl, K NO,, K 
Br and KI we find again the same rule; the larger number of mole- 
cules being most efficient in forming the so-called potassium larve, and 
so on down to the heaviest. The results hold only for salts of the same 
metal. 
Now since it is known from the work of H. De Vries and others 
that osmotic pressure is associated with the number of molecules in a 
given volume, increasing with diminishing molecular weight we find so 
close a similarity between the effects of salt upon larve and their 
osmotic action, that we may conclude, the author thinks, that these 
effects are due to their osmotic action. 
Thus the potassium larva is to be regarded as the result of dis- 
turbing those chemical processes which would have normally formed a 
lime skeleton, and this disturbance is by the removal of water osmoti- 
cally. Again the lithium larva may be regarded as due to some 
peculiar impermeability of sea urchin larval cells toward salt of 
lithium ; this produces strong osmotic pressure. The pressure is not 
regarded as working in a gross mechanical way, but rather as a stimu- 
lus that causes the larval cells to grow in an abnormal way. 7 
Though this explanation leaves no room for chemical changes as a 
source of change of form in these echinoderm experiments, yet the 
author is inclined to think that in some cases, as in the formation of 
galls, chemical changes of the protoplasm may cause the changed form 
that results. ae 
The application of this study in experimental embryology is that- 
the normal course of ontogeny is dependent upon the conditions of — 
osmotic pressure within and without the body. 
