PHYSIOLOGY OF ASCIDIA ATRA LESUEUR 295 
of our knowledge it is futile to attempt an explanation of their 
physiology. 
It has been tacitly assumed that chemical sense organs are 
capable of detecting substances in concentrations which fail to 
affect the ordinary cells of the body. This is largely because 
the effects on the sense. organs become evident through certain 
effectors, whereas the action on other tissues must be noted by 
special, indirect means on the cells themselves. When, however 
other tissues are studied, it is seen that they are influenced 
by concentrations of the same magnitude as sense cells. The 
effect of minute changes of the hydrogen and hydroxyl ions on 
the permeability of eggs and blood corpuscles need only be men- 
tioned. Acids and bases enter cells.in concentrations like those 
which stimulate animals. The poisoning effects of extremely 
low concentrations of alkaloids are also familiar. 
The modifications produced by these various substances are 
more or less the same for all cells and tissues: witness the simi- 
arity of effects produced on egg cells, sperm cells, fronds of algae, 
blood corpuscles, chromatophores, hearts, medusa bells and a 
host of others too numerous to mention. The concepts of ionic 
antagonism and salt balance apply not only to these tissues, but 
to sensory stimulation as well (Crozier, ’15b). It is therefore 
clear that chemical sensitivity is merely one of a large number of 
similar manifestations of the fundamental nature of cells 
The explanation which seems to me to account for all the 
phenomena o! this sensory activity, in Ascidia at least, is that the 
factor which primarily converts a group of cells nto chemical 
sense organs is not any special modification of their structure or 
sensitivity, but rather their connection, directly or indirectly, 
with an effector system. 
In this way the problem of the chemical sense of such aquatic 
forms is linked with the general problems of the physical chemis- 
try of cells and tissues. Our present knowledge, in this respect, 
of the chemical senses is, however, extremely meager. The time 
is therefore not ripe for any adequate explanation of the process 
in the receptor cell which results from the contact with a sub- 
stance in solution. 
