INTERNAL FACTORS OF REGENERATION IN ANIMALS 65 



level. We can picture to ourselves the same kind of process taking 

 place in the regeneration of the tail of a fish from an oblique surface. 

 The maximum rate of growth is found over that part of the cut-sur- 



FlG. 31. Planaria lugubris. A. Showing how worm was operated upon. B. A single head 

 regenerated at anterior cross-cut. It was united by a line of new tissue along the side of 

 the long half-piece with the new tissue at the anterior end of the short half-piece. The two 

 half-pieces reunited along the middle line. C. Two heads regenerated, one from each half 

 cross-cut The two half-pieces were kept apart along the middle line. 



face that is nearer the base of the tail (Fig. 40). At all other points 

 the growth is retarded, or held in check, and it can be shown that 

 the suppression is connected with the formation of the typical form 

 of the tail in the new part. If we cannot actually demonstrate at 

 present that this is due to some sort of tension between the different 

 parts which regulates the growth, we find, nevertheless, that it is by 

 means of some such idea as this that we can form a clearer concep- 

 tion of how such a relation of the parts to each other is established. 

 In a later chapter this subject will be dealt with more fully. 



THE INFLUENCE OF THE NUCLEUS ON REGENERATION 



The influence of the nucleus on the process of regeneration has 

 been shown in a number of unicellular forms. It was first observed 

 by Brandt in 1877 that pieces of Actinospfagrium eichhomii that con- 

 tain a nucleus assume the characteristic form, but pieces without a 

 nucleus fail to do so. Schmitz ('79) found that when the wall 

 of the many-celled siphonocladus is broken, the protoplasm rounds 

 up into balls, some of which contain one or more nuclei, while 

 F 



