334 



CAUSES OF VARIATION 



If an incision be made in the side of an actinian, tentacles 

 will arise at that point, and they will seize pieces of meat and 

 press them against the stem, whether a mouth was formed or 

 not 1 (see Fig. 43). 



If a planarian be split lengthwise into right and left halves, 

 regeneration takes place whether the halves be equal or unequal. 

 The completed worm will be at first both narrower and shorter 



than the normal, but with time and 

 feed it will equal or even exceed 

 the original. How the nerve cords 

 and genital ducts are formed anew, 

 especially from the piece so far to 

 one side as to include none of their 

 substance, is a mystery closely akin 

 to that of original differentiation 

 from the fertilized ovum. If the 

 slice be taken well to one side, no 

 head matter is included. In such 

 a case the head appears first at 

 the side of the piece, but later it 

 assumes its proper position. 



Regeneration from an oblique 

 surface. If the tail of a tadpole 

 though no mouth were present. be removed by an oblique cut, 



the regenerated tail will at first 



stand at right angles to the cut and obliquely to the axis of the 

 tail. As growth proceeds, however, the tail gradually swings 

 into line with the axis of the body. 



If the head of a planarian be removed by an oblique cut, it will 

 begin to regenerate at the forward part, making the head at first 

 stand at right angles to the cut, and not in line with the body. 



If a piece be taken from the middle of a planarian by two 

 parallel but oblique cuts (for example, running backward from 

 left to right), the head will begin to regenerate from the anterior 

 end at its forward (left) side, while the tail will begin at the 

 posterior extremity but on the opposite side, the final result being 

 complete restoration of the normal shape. 



1 Loeb, Physiology of the Brain, p. 52. 



FIG. 43. Lateral regeneration : a cut 

 in the side of an actinian produced 

 a clump of tentacles which would 

 seize a piece of meat and press it 

 against the side of the body, even 



