Remodeling of Old Tissues in Regeneration 135 



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. 107 



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 



10T Weismann : Das Keimplasma. P. 192. 



