Morcan: Hypotruesis oF Formative STurrs 207 
development of a head on one end of a piece of the worm, Lum- 
briculus, and of a tail on the other end of the piece, is due to fluids 
that flow forwards and backwards respectively, and act on head- 
germs at the anterior end of the piece, and on tail-germs at the 
posterior end. The accumulation of the head-stimulating fluid at 
the anterior end of the piece is supposed to awaken the germ of a 
head that lies at the anterior end. Conversely the tail-stimulating 
fluid, accumulating at the posterior end of the piece, awakens 
there the germs of the tail. It is unnecessary to point out that 
both the assumption of head- and tail-germs, and also the for- 
ward flow of certain kinds of substances, and the posterior flow 
of other kinds of substances are entirely fictitious assumptions, 
which from our modern point of view would be more difficult to 
account for than the phenomenon of regeneration itself. 
Sachs made use of the same idea, but in a very different form. 
He thought that the development of shoot-buds and of root-buds 
depended on the presence of certain substances in the plant. In 
order to account for the development of the shoots at or near the 
distal end of the piece, and of roots at the proximal end, he 
further postulated that the shoot-forming substances flow upwards, 
and the root-forming substances downwards —in response to the 
action of gravity. Vochting’s careful and elaborate series of ex- 
periments showed that the results cannot be explained so simply. 
He believes that an inner organization, or polarity, is the main 
factor in determining the result. Later Sachs also laid more 
emphasis than he had done at first on this innate factor. 
Goebel appears at times to advocate strongly the idea that the 
flow of substances in the plant is the cause of several of the 
phenomena of regeneration, At other times he applies the hypoth- 
esis in such a vague way and under so many forms, that depend 
on quite different principles, not clearly kept apart, that the ex- 
planation often appears quite confused. 
In his account of his experiments with seedling plants he makes 
little use of the hypothesis. On the other hand, in his beautiful 
experiments with cuttings of the stem of Bryophyllum he speaks 
out strongly in favor of the view that the results can be best ex- 
plained on the assumption of the flow of certain substances, or at 
least of their accumulation in the plant. He thinks that the results 
