INTRODUCTION 3 



complicated and entirely different from that in crystals. Let us 

 select as the simplest type of regeneration in living organisms 

 that of a sea anemone, Cerianthus membranaceus. This organism 

 may be described as a hollow cylindrical tube (Fig. 4), closed at 

 one end, the aboral, and open at the opposite, the oral end. 

 This latter end is surrounded by a row of tentacles. If we now 

 cut out a square piece, ahcd (Fig. 4), from the wall of the cylin- 

 drical tube, and keep it in sea water, the piece will live although 

 it is no longer capable of taking up food. In due time, which, 

 if the temperature is low, may be weeks or months, at one of the 



Figs. 



Fig. 4. Fig. 5. 



4 and 5. — Figure 4, diagrammatic view of an actinian from the wall 



of which a square piece, abed, is cut out. Figure 5, this piece forms tentacles 

 only on the side ab, which was originally oriented towards the oral pole. 



four sides of the piece, namely, ah (Fig. 5), which was oriented 

 towards the oral end of the animal, new tentacles will grow out, 

 but no growth of this kind will appear on the other three sides 

 of the piece. We may say that each isolated longitudinal ele- 

 ment of the body wall of Cerianthus possesses the power of grow- 

 ing into a tentacle at its oral end, but that no appreciable growth 

 in length occurs on anj^ of the other three sides. This is an 

 example of the so-called polar type of regeneration, which has 

 occupied the attention of the biologists.^ 



1 LoEB, J. : Untersuchungen zur physiologischen Morphologie der Tiere, II, 

 Wurzburg, 1891. 



