112 



SENESCENCE AND REJUVENESCENCE 



Anophthalmic. The anterior outgrowth of new tissue is vari- 

 able in form and without eyes, but contains a small, single, gangli- 

 onic mass, i.e., it is a rudimentary head (Figs. 22 A, 22 B). 



Headless- -The anterior outgrowth merely fills in the contracted 

 cut surface and does not extend beyond the contours of the margin; 

 the posterior outgrowth, however, is usually even longer than in 

 other pieces, but its differentiation proceeds very slowly and is 

 never completed as long as it is attached to the headless piece 

 (Fig. 23). 



The difference between the extremes of this series, the normal 

 and headless forms, in the degree of reorganization is very great, 



21 



u 



22 



23 



A V B 



FIGS. 21-23. Different degrees of reconstitution in Planar ia dorotocephala: 

 Fig. 21 A, B, teratomorphic forms; Fig. 22 A, B, anophthalmic forms; Fig. 23, 

 headless form. 



particularly in pieces from the postoral region (eg., a, Fig. 24). In 

 the development of a normal animal the anterior half or more of 

 such a piece undergoes extensive changes in giving rise to a pharyn- 

 geal and prepharyngeal region, and outgrowths of new tissue appear 

 at both ends. In the piece from this region which remains headless 

 no prepharyngeal or pharyngeal region arises, and changes are 

 limited to the longer outgrowth at the posterior end and the smaller 

 amount of new tissue at the anterior end. 



In the teratophthalmic, teratomorphic, and anophthalmic forms 

 the degree of reconstitutional change ranges from a little less than 

 in the normal animal to somewhat more than in the headless form. 

 Moreover the degree of reconstitution decreases somewhat as the 



