386 The American Naturalist. [April, 
Larger cells may move toward smaller and vice versa, or both to- 
ward one another. 
When three cells are concerned one may move toward another directly 
or may at first move as if in the resultant line of forces proceeding from 
each of the other two. 
Mass does not seem concerned in these movements for several cells 
in a group (not separated from one another in the teasing) do not act 
as a whole, but one of them may attract or else be attracted by some 
isolated cell lying near. 
Many cells may eventually come together and form a firm aggregate 
out of a scattered collection of isolated cells. 
It appears that these attractive movements take place between cells 
` of separate eggs as well as between the cells of the same egg. More- 
over, it was found that the cells of later stages, of the gastrula and 
young tadpole stage, may move. Thus cells that were forming the 
nervous system may, when isolated, round themselves off, become 
ameebird and even, in some cases, draw together till they touch. 
Besides the change of position hitherto mentioned there is a marked 
change of form. ‘In general two active cells protrude on the side to- 
wards the other cell so that they may be said to flow out towards one 
another to a certain extent. There is also considerable change in out- 
line, elongation and contraction of the cell while moving or while serv- 
ing as the centre of attraction or of movement for another cell. 
A cell may even divide while also moving towards another. 
The explanation of these complex movements of isolated cells in the 
frog embryo remains for the future, but provisionally the author refers 
them to the general class of movements brought about as the result of 
chemical action. That they are not simply physical, but results of life in 
the cells, the author seems to prove by careful examination of the 
sources of error and by controlling the conditions of experimentation. 
He would class these movements with those of sperm cells towards 
ova and of conjugating infusoria towards one another as cases of 
Cyrorropism ; he pictures to himself a chemical or chemotactic source 
for the movements by supposing that the cells secrete chemical sub- 
tances that effect other cells so as to direct their movement as well as 
to incite it. This movement under the stimulus of adjoining chemicals 
would differ from that observed by Pfeffer, in that here the cell does 
not move towards the region of greatest concentration of substance, but, 
in that it moves to another cell and thus into the field filled by sub- 
stances from two cells, towards the region where the substance is least 
dilute. 
