516 JULIA B. PLATT. 
pressure. The circumstance which gives occasion for this 
comment is connected with the folding off of the alimentary 
canal, and Wilson ascribes the apparently independent action 
of the cells to inherited tendency to follow ancestral lines of 
migration. The action of cells, however, in the path of the 
lateral line in Necturus seems the more peculiar since we cannot 
ascribe it altogether to heredity, because of the irregularity 
with which the sense-organs are formed. In asegment on one 
side of the embryo a sense-organ often appears that is omitted 
on the other side. Now one, now two segments are omitted. 
Here two, there three sense-organs are allotted to a given seg- 
ment. The inherited tendency is evidently one that allows 
wide range to individual variation, and this fact renders the 
independent action of individual cells most striking. 
I am convinced that we shall never have even approximately 
accurate knowledgé of the course of vertebrate development 
until we are by some means enabled to follow the migration of 
individual cells. We recognise the advancing mass, or the 
elongating cord, but shut our eyes to the fact that cell after cell 
moves on its independent mission, wandering alone—who 
knows how far? or in what direction ? 
Merkel (28), in commenting on Mall’s (27) statement that 
the direction of the transmission of an impulse is already 
determined by the position of the cell in the ectoderm, the 
receptive pole being that originally on the surface of the body, 
says (loc. cit., pp. 299, 300) that it would be interesting to 
know if this view be really of general validity, for Merkel 
finds it conceivable that the direction of a nerve-current 
might change; at least “such a possibility must be first ex- 
cluded before one can speak with certainty even in regard to 
the retina.” The migration of cells in the lateral line, which 
I have just described, seems to me to demonstrate that the 
law Mall states is not generally applicable, for the impulse 
which induces three of six cells lying in a continuous line, and 
equally exposed to the surrounding water, to migrate while 
the rest remain, must surely be received from within. A 
transmitting pole has therefore become a receiving pole, and 
