828 
ROSS GRANVILLE HARRISON 
Schaeppi. He has urged that in the lymph experiments the ex- 
cised fragments may have inckided protoplasmic bridges such as 
he has figured from Lepidosiren embryos, and that these might 
differentiate later into nerve fibers. In reply to this it may be 
pointed out that the embryos used in the experiments were of a 
relatively younger stage than those Kerr has in mind and con- 
tained no protoplasmic connections of the kind figured, as serial 
sections readily show.^^ Furthermore the pieces of the tissue were 
cut out with sharp scissors, sometimes by a single clean cut be- 
tween the neural tube and the notochord, or in other cases, after 
the neural tube had been separated by the scissors from the muscle 
plates. All protoplasmic filaments must have been severed by 
this mode of cutting, and while, of course, short ends may possibly 
have been left attached to the cells, and have remained invisible 
in the preparations, they would by no means be able to account 
for the great length — over a millimeter in some cases — attained 
by the fibers. Besides, what is much more conclusive, the end 
of the fiber is an actively motile mass of protoplasm. 
Held's ('09) criticism takes a different turn. He admits^^ that 
the fibers seen in the lymph are really the beginnings of nerves 
(Ansatze einer Nervenbildung), though on the next page he main- 
tains — on what grounds it is not clear — that if the nerves of an 
embryo did develop exclusively in the manner described by me, 
Welt will mich denn davon iiberzeugen, dass das, was unter dem Mikroskope als 
das Ende einer Faser erscheint, nun wirklich in Tat und Wahrheit das Ende ist?" 
In doubting that such ends, in which motion and extension can be observed with 
absolute certainty, are actual ends, it seems to me that the supporters of the pro- 
toplasmic bridge theory are pushing skepticism about one thing beyond the 
utmost limit, and at the same time are placing an equally unbounded faith in the 
invisible. One cannot but think of the epithet "noli me tangere" by which 
Hensen designated the outgrowth theory, and wonder if it might not be applied 
with much greater appropriateness to the plasmodesm hypothesis. Could the 
botanist, if pressed for an absolutely rigorous proof that the roots of a plant grow 
out from the radicle and are not preformed in the soil, give an answer based on 
evidence of any different kind from that given here for the outgrowth theory of 
nerves? 
To my mind the protoplasmic bridges which Kerr figures are simply the pro- 
cesses that have grown out from the cells in the ventral part of the cord. 
14 Op. cit., p. 260. 
