Remodeling of Old Tissues in Regeneration 1 35 
soblastic cells made them become mesoblastic cells.” And 
Weismann finally calls into question the incontestability 
of these facts, just because such a cell determination de- 
pendent upon contiguity would upset at once his whole 
theory of preformation.!°7 
But there are also many cases of regeneration proper 
in which one has a remodeling of old tissues into new 
tissues that are quite different, and they constitute 
phenomena which are analogous in this respect with post- 
generation. As an example may be mentioned the 
regeneration of Planaria maculata. 
Fragments of this worm obtained by two transverse 
sections regenerate the head and the tail by producing 
new cells. But after their formation, this head and this 
tail do not grow any further, but the entire subsequent 
growth in length of the body takes place in the older more 
pigmented parts, so that the normal relative proportions 
of the planaria are restored simply by a remodeling of the 
older tissues. “The fragment of the worm reacquires its 
normal form but not through the addition of new tissue 
at the anterior and posterior extremities, except to a very 
small extent. The transformation is produced chiefly in 
the old tissue after the head and tail are developed. Thus 
we find here not only the capacity of regeneration but 
also a subsequent self-regulation by means of which the 
normal relations of the parts characteristic for the species 
become re-established.” But that is not all. For in an- 
imals regenerated from lateral fragments, the longitudinal 
axis of the new worm is found often in the older tissue, 
so that one portion of the old material which was in the 
right side of the old animal becomes now part of the left 
2°™Weismann: Das Keimplasma. P. 192. 
